I have a 3G on T-mobile using iOS 3.1.12 (I know I live in the stone ages) but I managed to get MMS to work on my phone by going into the General Settings --> General --> Network --> Cellular Data Network and enter the following info:
MMS
APN: epc.tmobile.com
Username:
Password:
MMSC:
http://mms.msg.eng-t-mobile.com/mms/wapenc
MMS Proxy: 216.155.165.50:8080
MMS Max Message Size: 104576
MMS UA Prof URL:
http://www.apple.com/mms/uaprof.rdf
Since you're on T-mobile, you must be jailbroken - if you go into Cydia, I think there is an MMS T-mobile fix available now. You might try that if the above settings don't work for you. HTH!