One possible solution to the problem of no Mail.app on the iPod touch would be to sell it as an extra. It would keep AT&T sweet (anyone who would pay for Mail on the touch is likely not going to get an iPhone), and it would increase the functionality of the touch in the one area I think it's really lacking.
So... the question is, who would pay for Mail on their touch?
Personally, I'd be willing to pay $20/£10 for it—the usefulness of it makes it worth at least that much.