It's true, as Mike points out, both Obama and Bush are proposing similar-sounding CAFE increases. The main difference, I think, is that the White House has given every indication that it intends to be much more, um, "flexible" about things. Here's a telling caveat from the president's State of the Union address, which estimated that his CAFE plan would reduce gasoline consumption by 8.5 billion gallons in 2017:
These amounts are based on an assumption that on average, fuel efficiency standards for both light trucks and passenger cars are increased 4 percent per year.... Given the changing nature of the marketplace for both cars and light trucks, the Secretary of Transportation will determine the actual standard and fuel savings in a flexible rulemaking process.
Under Bush's plan, the Transportation Department would set the rules for cars and light trucks (currently it just handles the latter), rather than Congress. And the 4 percent annual increase would be a vague goal, not the actual law. Indeed, when Nicole Nason, head of NHTSA testified before Congress in March, she couldn't say what target the agency would eventually set. Senate Democrats suspected they were being suckered. Not surprisingly, U.S. automakers are lining up behind Bush's proposal. Now, the Senate bill has more than its share of loopholes (and, in its present form, still gives the NHTSA far too much leeway), but if those can be tightened, it will almost certainly prove more effective.
Update: On the other hand, if you think that Obama's plan only differs from Bush's in degree, and not kind, you might want to check out Chris Dodd's proposal to increase fuel-economy standards to a whopping 50 miles per gallon.