I'm using 2G iPhone unlocked/jailbroken with PwnageTool. My carrier is T-Mobile and I have the $5.99 T-Zone or whatever it's called.

My regular APN for cellular data is epc.tmobile.com

For MMS
APN: [blank]
Username: [blank]
Password: [blank]
MMSC: mms.msg.eng.t-mobile.com/mms/wapenc
MMS Proxy: 216.155.165.50:8080

Like others, the only problem is when receiving MMS, the photos are super compressed. This happened with SwirlyMMS also but they allowed you to switch the user agents. I suppose this is better than nothing.

I hope this helps other users and thanks to those who made this possible.