ok heres the deal guy... easy fix for mms on 3.1.2 on 3g iphone (i dont know about the other phones cus i only have a 3G)
what i did to fix my issue was:
Clear all network settings(general>reset>reset network settings)
let phone reboot
once rebooted navigate through safari to "help.benm.at/help.php"
once on the site click on Mobile Configs then USA then Tmobile
It will ask you to install a package...install it
then your MMS settings will be available in Settings>General>Network>Cellular Data Network
Then input these values:
APN: wap.voicestream.com
No Username/pass
MMSC: 216.155.174.84/servlets/mms
MMS: Proxy 216.155.165.50:8080
MMS Max Message Size: BLANK (Make sure to leave this field blank)
MMS UA Prof URL:
http://www.apple.com/mms/uaprof.rdf
Make sure you enter all settings CORRECTLY and don't copy and paste or it wont work
after you have entered your settings give your phone a reboot and your camera icon should be there and you should be able to send mms. This is how it worked for me.