When hundreds of experts gather in San Jose this week for the country’s first and biggest plug-in hybrid conference, the tone of the gathering will reveal a dramatic change.

Automakers, utility representatives and policy-makers will be talking when, not if, at Plug-in 2008.

Asked whether plug-in hybrids, also known as plug-in hybrid electric vehicles, or PHEVs, are inevitable as production vehicles, Mark Duvall of the Electric Power Research Institute (EPRI) in Palo Alto answered with cautious optimism that sounded more optimistic than guarded.

“I would never say 100 percent until I can kick the tires myself, but I’m as optimistic as I’ve even been in 15 or so years with this subject,” Duvall said.

Like traditional hybrids, a plug-in hybrid has both batteries and a gasoline engine, and can operate on either or both. Unlike traditional hybrids, plug-ins have larger battery packs and a plug that allows for household charging, meaning they can travel much farther on electricity. And while a hybrid such as the Prius can only go a few miles at low speeds solely on electricity, plug-in advocates envision vehicles with 20 to 40 miles of battery-only transportation range.

Duvall is one of the speakers at Plug-in 2008, which runs today through Thursday at the San Jose McEnery Convention Center, and is organized by a host of supporters, including the Silicon Valley Leadership Group, Pacific Gas & Electric and EPRI. The event isn’t open to the public but does include a public session Tuesday night.

Representatives from General Motors, Toyota and Ford will be on hand. So will officials from PG&E and Southern California Edison; battery-makers; government researchers; and plug-in advocates.

And former Intel Chief Executive and Chairman Andy Grove, who writes and speaks publicly about his fondness for plug-ins, which he calls dual-fuel cars, will give the keynote address Tuesday.

It’s an important topic in Silicon Valley, one of the largest markets for the Toyota Prius, the best-selling hybrid and a poster child for those embracing green living.

“We’re a very receptive market for new technologies that can make a substantial difference regarding climate change,” said Laura Stuchinsky, who also will speak at the conference. Until recently, she headed up the Silicon Valley Leadership Group push to promote production of plug-in hybrids. She’s now the sustainability officer for the city of San Jose’s transportation department.

At this point, the consensus is that GM and Toyota will be offering plug-in hybrids by late 2010 or early 2011. Other automakers are expected to follow.

Media reports suggest that GM will unveil a production version of its Chevrolet Volt, which it describes as a range-extended electric car, as soon as September. The Detroit automaker says the Volt will have a small gasoline engine to extend the range of the vehicle’s lithium-ion battery pack. It also says it can be charged via a 110-volt plug.

Meanwhile, Toyota is currently testing plug-in versions of the Prius in France, Japan and the United States. About a half-dozen of the cars are being evaluated at Toyota’s U.S. headquarters in Torrance as well as by researchers at the University of California campuses in Berkeley and Irvine, said Jaycie Chitwood, senior strategic planner for Toyota’s advanced technology vehicle group.

Those who have driven the car have had positive reactions, Chitwood said, although they question why the vehicle can only travel seven miles on electricity. The car’s next-generation battery will improve on that, she said.

“It’s very smooth, seamless,” Chitwood said. “You feel like you’re driving a Prius plus.”

But with the increasing hype about the potential of plug-ins, Toyota feels the need to reign in expectations a bit, she said. “We want to make sure people understand what plug-ins are, and what they aren’t,” she said. “We still believe that hybrids will be our core technology going forward.”

If Felix Kramer has anything to do with it, plug-ins will be a way of life. Kramer founded the California Car Initiative, better known for its Web URL, CalCars.org, to promote plug-in hybrids. Based in Palo Alto, Kramer drives a converted Prius emblazoned with “100+ MPG” on its sides. The movement toward the vehicles has reached “a consolidation moment,” Kramer said.

From 2010 to 2012, he expects to see an increasing number of plug-in hybrids from big car companies on the road. “The carmakers are in a race,” Kramer said.

“All the important constituents agree this is a good idea, and they’re trying to figure out how to make it happen on a large scale and rapidly.”