How Green is your Hybrid?
When comparing the relative "greeness" of hybrid cars - although no car can truly be called green, except maybe for this one and some non mass-produced prototypes - most people will look at the fuel efficiency figures (the mpg ratings). This can be misleading, or at least, not give the whole picture. Emissions are also an important factor. For example, the second generation Toyota Prius and the Ford Escape Hybrid are rated SULEV (Super Ultra Low Emission Vehicle) while the Honda Civic hybrid (except for the "lean burn" model, which to our knowledge is not widely available) and Accord hybrid are rated ULEV (Ultra Low Emissions Vehicle). The difference between the two is mostly in the CO and NOx emissions; ULEV, compared to SULEV, is allowed to produce about twice as much CO and a bit more than three times as much NOx (which is a main ingredient of smog).
Some people will argue that ULEV is clean enough, and to back up this claim they will mention how much cleaner our cars are than the cars from the 60s and 70s. That's correct, but it is also true that the average fuel efficiency in the U.S. has decreased in the past 20-25 years, in large part due to the popularity of SUVs, and that while current cars produce much less polluting emissions than the cars of yore, we are not quite there yet and there are a lot more vehicles on the roads of the U.S. and of the rest of the world (especially Asia and South-America) than ever before. Aiming for the cleanest possible car is justified, and in that area those with a SULEV rating like the Prius have an advantage over the ULEV ones like the Honda hybrids that the mpg figures alone don't show. If we wanted to be really mean to Honda, we'd add that there are already dozens of regular gas-powered vehicles that are rated ULEV (hat tip to John1701a) and that a hybrid design that doesn't help reduce emissions past that point is only part-successful in its environmental aspirations.
Of course, emissions are only one of many factors; on one hand, we would be hard-pressed to recommend a SULEV Escape hybrid that gets 28 mpg over a ULEV Civic hybrid that gets 46 mpg, but on the other, these two are rarely competing against each other in the real world. An Escape hybrid that replaces a Suburban would make a bigger difference than a Civic hybrid that replaces a regular Civic. The best scenario, when practical, is of course to combine low emissions with high mileage (well, the very best scenario is that you walk or cycle to your destination, but sometimes that just not possible - except for that guy, maybe).