Here's a site that uses actual driving data from its members to arrive at average MPG estimates for hybrid cars:
Very interesting stuff. I also saw some calculations a guy did on the break-even point for buying the prius over a corolla. He thinks a corolla can achieve 35mpg without hybrid technology and compared the base sticker price with that of the hybrid ($15.5k for corolla and $21k for prius). He assumes 45mpg for the prius and $3/gallon gas. Here's his formula: (x/35-x/45) = (21000-15500)/3. This resulted in around a 280,000 mile pay-off for buying the hybrid not taking into consideration differences in maintenance.
Doesn't seem to be compelling, but buying a hybrid is just so cool.