I just did. At&t US that is...

Heres what i did...

Download fAPN.
Create a new profile.. I called mine Att mms
Go in to your network connections and edit the one u just made...
It should say...
Con. Name: Att mms
Con. Type: GPRS
NEXT
Access P. Name: Att mms
Username: WAP@CINGULARGPRS.COM
Password: CINGULAR1
NEXT
ADVANCED
Check Use proxy and under HTTP proxy type
wireless.cingular.com
Port: 80
Then save it and press Finish

Now open fMMS...
At the top click Configuration
Type in the APN you used earlier (Caps sensitive)
and fill in the rest like so...
APN: Att mms
MMSC: http://mmsc.cingular.com
Resize image: 300
Phone #: 3033033030

Thats what got my MMS to work!
Good Luck!
-Tyler