Same problem for me, on an iphone 4s.
I called about two weeks ago when I was having this issue and was told to change the settings to these:
• MMSC: http://mmsc.cingular.com
• MMS Proxy: mms2.tracfone.com:80
It worked great for a few days, then right back to the same old problems. No data whatsoever if I'm away from a wifi hotspot.