Treehugger concludes that hybrid-drive cars' shade of greenness varies considerably, and that such greenness is still a relative measure. (Just like a guy we knew in college, they all still, drink refined beverages and fart particulate matter.) Still, some models are more eco-minded than others, based on how thirsty they are and exactly how much particulate matter they leave behind. That's the short answer.
How Green is Your Hybrid [Treehugger]
Related:
New York Auto Show: Lexus 450h [internal]