You can just abuse standard DNS for this... just stick a TXT record in a well known place eg. _bitcoin,

So a client sending bitcoin to foo.com just looks up _bitcoin.foo.com and reads off the address from there.

The problem is (a) modifying DNS is easy for geeks, not so easy for grandma.  And geeks can already use bitcoin addresses.. So it doesn't really get us very far along the usability stakes, and (b) as it stands it's not good at multiple users per domain.

Now if someone can find a away of mapping email to bitcoin address without introducing massive amounts of infrastructure or a central database, then that'd be a good solution (as easy as paypal, without the fees.  People would go for that).