I know when I get an MMS sent from a friend AT&T sends me an SMS with a link and password to their server and I can see it in the iPhone's browser by touching on the link in the SMS. The biggest problem with that is you need to write down the code somewhere first because there is no copy-n-paste yet.
as for email, AT&T does have their own email its:
[10-digit phone number]@txt.att.net
Example:
2125551212@txt.att.net
That address is more for sending SMS from your email to them. I don't know anyone that actually uses this to check email. Most people use their actual email address as the iPhone can check that directly.