The recent release of the Beatles' music on iTunes, coupled with the anniversary of John Lennon's tragic death in New York City 30 years ago this past Wednesday, has brought on a wave of Beatles nostalgia. For so many of my generation, growing up in the 1960s and 1970s, Lennon was a hero, not just for his music but for his fearless activism against the Vietnam War.

Is there a celebrity activist today who matches Lennon's impact and appeal? The closest counterpart to Lennon now is U2's Bono, another transcendent musical talent championing another cause: the battle against global poverty. But there is a fundamental difference between Lennon's activism and Bono's, and it underscores the sad evolution of celebrity activism in recent years.

Lennon was a rebel. Bono is not.

Lennon's protests against the war in Vietnam so threatened the U.S. government that he was hounded by the FBI, police and immigration authorities. He was a moral crusader who challenged leaders whom he thought were doing wrong. Bono, by contrast, has become a sort of celebrity policy expert, supporting specific technical solutions to global poverty. He does not challenge power but rather embraces it; he is more likely to appear in photo ops with international political leaders - or to travel through Africa with a Treasury secretary - than he is to call them out in a meaningful way.

There is something inherently noble about the celebrity dissident, but there is something slightly ridiculous about the celebrity wonk.

Lennon was no Johnny-come-lately to the antiwar movement. As early as 1966, during the Beatles' American tour, he answered a reporter's question about Vietnam, much to the consternation of the band's business manager. "We just don't like it. We don't like war," Lennon said simply. And when he married Yoko Ono in 1969, they used their honeymoon to stage two seriocomic "Bed-Ins" to publicize the antiwar cause.

Lennon also merged his activism and his music: In 1969, "Give Peace a Chance" became the anthem of the movement after half a million people sung along at a huge demonstration at the Washington Monument. That same year he sent back an award he had received a few years earlier from the queen of England, in protest of British support for the Vietnam War. After moving to New York in 1971, he continued his high-profile opposition to the war, and two more songs released that year - "Imagine" and "Happy Xmas (War Is Over)" - expanded his antiwar repertoire.

Lennon paid a price for his activities. We now know from subsequent Freedom of Information Act releases that the FBI monitored and harassed him. In 1971, President Richard Nixon set in motion a four-year effort to deport him, which failed after the political tide in America turned against the war.

In this role, Lennon was continuing a venerable tradition: the celebrity as a crusader against the wrongs committed by those in power. In the 19th century, the celebrity activists were not musicians but writers. Charles Dickens, Ralph Waldo Emerson, Henry David Thoreau and other authors loudly supported the abolitionist crusade against slavery. Harriet Beecher Stowe went further and wrote "Uncle Tom's Cabin" to boost the anti-slavery cause - a sort of 19th-century equivalent of "Imagine."

Mark Twain denounced American imperialism and atrocities in the 1898-1902 war against Spain and Filipino independence fighters, publishing his savage satirical essay "To the Person Sitting in Darkness" in 1901. In the imperialist claim to spread "civilization," he detected "two kinds of Civilization - one for home consumption and one for the heathen market." Twain also saw "two Americas: one that sets the captive free, and one that takes a once-captive's new freedom away from him . . . then kills him to get his land." Other Twain essays on the same issue were so politically toxic that he could not get them published during his lifetime.

Alas, today's celebrities seldom challenge power in the manner of Twain or Lennon. Bono's signature effort involves the Millennium Development Goals campaign, a United Nations-sponsored initiative to achieve eight anti-poverty goals by 2015. The campaign stresses that 189 world leaders have endorsed the targets to reduce poverty and hunger and to improve health by the deadline.

In the course of his activism, Bono had regular photo-ops and lunches with President George W. Bush, giving Bush a much-needed publicity boost on U.S. foreign aid and on his campaigns against AIDS. For example, the singer appeared onstage with Bush at the Inter-American Development Bank in Washington in 2002 as the president pledged a $5 billion increase in foreign aid. In May of that year, Bono even toured Africa with Bush's first Treasury secretary, Paul O'Neill, fully aware that the administration was capitalizing on his celebrity.