Few words in the Anglo-American academic lexicon cause more confusion on the Anglo side than "commencement". It's what the Americans call "graduation". And it takes place, midsummer, between the end of May and the beginning of June. For four weeks, there are more mortarboards than skateboards on campus.

The word "commencement" invariably wrinkles British brows. The psychology's wrong. We think of graduation as something concluded. Americans, more optimistic by nature, see it as life's starting blocks. To instill that up and at 'em spirit, American universities and colleges recruit "commencement speakers". If you follow this back to colonial times, one can see it as originating in the inspiring sermon that accompanied the solemn ceremony of academic completion. The first commencement speakers were all clergymen.

The earliest speaker I have identified was the Rev LT Chamberlain, who gave the address at the University of Connecticut, then an agricultural school, in May 1882. It was a somewhat anti-climactic affair. None of the students had completed the requirements for graduation and no diplomas were awarded. After the ceremony, students conducted laboratory practice demonstrations.

It's different today. Commencement is the splashiest public event of the academic calendar. But shreds of that pioneer piety and sermonising still remain in the ten-minute address. Bill Clinton (who's doing the rounds of six campuses this summer, Hillary - preoccupied with fund-raising - is doing only two) puts it this way:

"I always urge them to have dreams that are both timely and timeless, to try to live their dreams knowing that life's real regrets are more rooted in not trying than in not succeeding, and to find joy in the effort."

Dream the impossible dream - where have I heard that schmaltz before?

Addressing the most venerable of African-American campuses, Howard University, on May 12, Oprah Winfrey, "commencement oratrix", told her ecstatically cheering, begowned audience: "I stand here as a symbol of what is possible when you believe in the dream of your own life."

With that sublimely meaningless platitude Dr Winfrey, who has honorary degrees like Imelda Marcos had shoes, received another PhD in humanities. She may also have received a hefty fee. Or not. Universities are chary about revealing how much they pay their speakers: it rather takes the bloom off the platitude.

Over the last three decades, commencement speaking has become big business. Katie Couric, a TV news anchor, is speaking this year on June 3, at Williams College, in Massachusetts. It's not reported how much they're coughing up. But last year Dr Couric cost Oklahoma a cool $110,000 (£55,360). According to the Washington Post, she informed the new graduates: "Not only do I like spunk, but you've got to have it." Worth a hundred grand of anyone's money.

The going rate for top-drawer speakers is said to be half Couric's. But for that you probably won't get the stars: Bill Cosby, Maya Angelou, or the late Kurt Vonnegut. Cosby, who's done 21 commencements in the last five years, probably needs a small warehouse for his scrolls and plaques. And a Brinks van for his payments.

The hypocrisy beneath the commencement flummery can be breathtaking. Barack Obama, for example, in his address at Southern New Hampshire University on May 19, enjoined the 1,000 new graduates to "resist the money culture". This from a candidate who, for the previous six months, had been shaking down donors for his presidential bid as single-mindedly as Paulie Walnuts doing the rounds for Big Tony. Between January and the end of April, Obama raised $25m from 100,000 supporters. Not much resistance to the money culture there.

John Edwards, Obama's rival, spoke to students at the University of California at Davis in January on the rousing topic, "Poverty, the Great Moral Issue Facing America." His fee? $55,000 (£27,700). Edwards gave the commencement address at New England College this May. "The irresistible power of America lies in your hands," he told the new graduates. How much flowed back into his hands is unreported.

Perhaps nothing. Nor, I suspect, will the owner of Microsoft expect any addition to his trillions this summer. Bill Gates is speaking at Harvard this June. He could buy the place - and it must be a good feeling. He's the university's richest dropout, having left in his second year to make his first million. Only schmucks hang around for graduation. Somehow, though, I expect there will be more platitudes about dreaming the dream.

Politicians love the commencement podium. Even if, like Bill Clinton, they ostentatiously decline money, there's always a payoff. They look distinguished in their academic finery, the graduands and their proud parents are the friendliest of audiences. And, most importantly, commencement speeches are reported nationally.

George W Bush rotates his commencement duties among the military academies. This year on May 23, it was the US Coast Guard academy, and he used the event to stress the ongoing threat of the evil ones: stand by your boats, men, Osama is coming back. Dick Cheney spoke at Brigham Young University, the Mormon institution generally regarded as the most conservative university in the nation. It still imposes strict regulations on "modesty attire". Alas, even in this Republican stronghold, there were campus demonstrators... modestly attired.

Generally speaking, high-ranking government politicians, still in office, are not welcome in campuses that - unlike the US Coast Guard academy - offer easy access to the public. As his popularity sagged, in the early 1990s, George H Bush did a round of commencement addresses. But the vast presidential security apparatus (bulletproof screens on three sides of the podium) rather took the fun out of the event. As their offspring went up for their scroll, parents were uneasy at the thought that their pride-and-joy was in the crosshairs of half a dozen highly trained marksmen.

Who chooses? It used to be the senior faculty. Increasingly it's student advisory committees which have the clout. When Rice University, in 2005, decided for the first time to have a serving woman professor on their own staff give the commencement address, the students went ballistic. They wanted Lance Armstrong, Robin Williams, or Bill Gates. Bill Cosby gave the address in 2002. That had gone down very well.

The institution's argument that "With a typical commencement speaker's fee of between $25,000 and $35,000, you could hire an assistant professor for a year" cut no ice whatsoever with graduating seniors. "If I had wanted to hear her speak," snarled one, "I could have gone to her class." This year, Rice has John Doerr - described as "the single best venture capitalist in the world." That's more like it.

Commencement speaking hasn't caught on in the UK, and perhaps never will. But, rest assured, one Briton (and probably his wife) will be doing the circuit next year. Look to your laurels, Cosby: the Blairs are coming.

· This article was amended on Friday June 1 2007.

We wrongly stated that Bill Cosby gave the commencement address at Rice University "two years running, in 2001-02." Mr. Cosby has presented only one commencement address at Rice - in 2002. Civil rights attorney Morris Dees gave the commencement address in 2001. This has been corrected.