I think tax credits have traditionally been instituted to help someone when they incur extra expenses, like having a child, for example. By not owning a motor vehicle, we're usually saving money by choice, and therefore a tax incentive isn't necessary. The whole energy/environmental aspect of living car-free isn't even considered.
That, I believe, is the old school of thought.
In the new school of thought, there ought to be tax incentives for living car free, especially since there are already tax credits going to those who drive hybrids. And like I've always said, the SUV driver who only uses the thing a few dozen times a year is more favorable in my book than the hybrid driver who commutes 50 miles round trip five days a week.
I like the idea of an official DMV document. That seems to be the only legit way to prove car freedom. I think I'm going to write to my congressman about this.