If you use cruise control more on the highways, you will get an additional 2-3 mpg which will more than make up for any feel good savings. As far as greenhouse gases go, we all better start eating more meat so that there are less cows producing less methane in their farts.
I see no credence in the fears of global warming. The evidence simply isn't there. It's like a fear generator.
You're right about cow farts... can you believe the government spent untold MILLIONS of dollars on studies to measure the effects of cow gas? And how about the evolution nut who climed that dinosaurs went extinct due to their own flatulence (they couldn't take the heat)? I love the boneheads of athiesm and evolution... they are funny to watch.
I personally wouldn't buy a hybrid now, I'd wait 10 years when the technology is refined, and the car will be an actual gas electic hybrid, where you can recharge the battery from an outside energy source. Electic hybrids are a great idea, especially for cars that are stop and go, but its only the first generation of these things.
Plus me studying electrical engineering, I wouldn't trust myself with a hybrid. I know I'd attempt to modify it to take a charge from an outside source, ruining my warranty, and possibly the whole car!