We have recently had a huge server crash and lost most of our data. Our Cydia repo is currently down, but you can manually fix your MMS with the Simple Mobile MMS Fix for iPhone below. This fix should work on iOS firmware 4.0 to 4.2.1+.
UPDATE 2: If your phone is not jailbroken follow this guide to get mms working on a non jailbroken iPhone: https://leimobile.com/simple-mobile-iphone-mms-settings-for-ios-5/
UPDATE:Please add our Cydia repo to find the iPhone MMS Fix for Simple Mobile.
Our Repo is http://beta.leimobile.com/repo/
Once you install the fix you will need to reboot your phone and go into settings->general->network and make sure the APN is correct.
If you are using the $40 TalkTextILD plan, you cannot use MMS as a data plan is required. When your current plan expires, upgrade your plan to one of the data capable plans – 50MB or 150MB – that Simple Mobile offers.
To install you will need to SSH into your iPhone. The first thing you will want to do is download OpenSSH from Cydia. Next you will want to follow the guide to using WinSCP for iPhone.
You will need to place the override plist for Simple Mobile in /var/mobile/Library/Preferences which you can download here: com.apple.mms_override.plist Make sure you REMOVE SIMPLE from the file name. So save the file as com.apple.mms_override.plist
SIMPLE MOBILE SETTINGS
Mms proxy: 216.155.165.050
Mms max: 1048576
Mms ua prof URL: http://www.apple.com/mms/uaprof.rdf