Simple Tutorial to Hack for Free MMS on the iPhone 3G running 3.0 with T-Mobile. Note that this hack only works for T-Mobile on the iPhone 3G.
First, go to Settings > General > Network > Cellular Data Network.
Add the following in MMS field:
* APN: wap.voicestream.com
* Username: leave blank
* Password: leave blank
* MMSC: 188.8.131.52/servlets/mms
* MMS Proxy: 184.108.40.206:8080
* MMS Max Message Size: leave blank
* MMS UA Prof URL: leave blank
Again, this only works if you have T-Mobile. It does not work with other carriers.