[woodruffw]

I'm not sure whether it's universally supported, but AT&T at the least actually does support responses. I can text any email address from iMessage and receive an email via SMTP from [e-mail], and responding to that email also delivers correctly (and is even threaded).

Edit: One funny thing is that iMessage will detect gateway traffic from/to email addresses that are tied to iMessage accounts, and put those into a "blue bubble" chat instead of the normal SMS/MMS "green bubble" chat.