Just thought I'd throw this link up on a company that actually supplies thermo electric generators, they are planning to release one for auto exhausts in 2012, at bottom of page.
Thermoelectric Generators Wholesale (All Products)
There is another company out there two, I occassionally look them up to see how they are progressing with their products.
Still think a practical affordable unit will be a few years away, but it is moving forward, which is good.