If only it were that simple. We know that a petrol burning motor can achieve an extremely high level of volumetric efficiency, with low emissions. Witness the Honda ULEV engines. Drive one in LA, and the tailpipe is cleaner than the intake. I know emissions with hydrogen power is simply water, but how well does it burn? Alcohol and propane engines are in use, but they make very little power compared to a petrol motor. Let's say you were able to convert a Corvette to hydrogen. Yes, it would be cleaner burning, but it might only produce 80HP. I don't see conversions any time soon.

[edit] It's been a while since I've bothered reading up on this, so if someone would like to enlighten me on the properties of hydrogen gas, and its potential as an alternate fuel, I'd be happy to read what you have to post.