To all who are still trying to get MMS on iOS 4 working. I finally got MMS working on my iPhone 3G using the following method and settings:
I jailbroke using redsnow 0.95.5b5-5.
I unlocked using ultrasnow in Cydia.
I installed the T-Mobile 4.0 MMS Fix in Cydia from the following repository, http://beta.leimobile.com/repo/
Lastly, I am using the following settings:
Enable 3G: Off
Cellular Data: On
Data Roaming: On
Internet Tethering: Off
MMS Proxy: 188.8.131.52:8080
MMS Max Message Size: 1048576
MMS UA Prof URL: http://www.apple.com/mms/uaprof.rdf
Hope this helps everyone that was in the same boat as me.