I was looking for exactly the same thing and found Canon MF4690PL. Its It does exactly what you want: with a push of a button you can scan directly to an e-mail address (or to an FTP or a shared folder). Without a PC. It has 7 one-touch buttons (for 7 email addresses) but you can use more (coded dial) with a little bit of extra button pushing work that still beats PC-based solutions any time :-)
Also, Canon's UFRII drivers are much more terminal friendlier than HP's drivers. I found that one out as a "side-effect" ;-)
It's the only sub-$500 product that supports that feature I've found so far.