Ive seen a few threads about this, as well as several on the TMo boards and other boards about people not being able to send or receive MMS over wifi after the ICS update. I also had this issue and called TMo. I spoke to their 2nd level support, and the solution per Samsung is first send a picture to yourself via gmail (the gmail acct associated with your phone) while on wifi. After you receive it, you should be able to send and recieve MMS over wifi from there on out.
It worked for me, so I thought Id share with you. I cant speak for custom ROMS as Im on stock rooted.
edit: the symptoms I was having was I would send an MMS, and it would sit at sending. If I received an MMS, I got the notification of a new text with a download button. Hit that, and it sits at download. FYI I just checked my wifes SGS2 and she's on Darkside 2.7 and had the same issue. I perfomed the fix above, and now it works. So it appears this fixes the problem on GB as well.
Last edited by blackangst; 21st June 2012 at 09:08 PM.