Here are the settings I have found to work on the iPhone 3GS on the GCI (www.gci.com) Alaska cellular network. Note that my phone is running 3.1.2 installed using a Pwnagetool firmware bundle and 5.11 baseband w/ the Blacksn0w patch to carrier unlock it.

Note that after setting the settings for MMS, you should reboot the phone, then test.

Settings > General > Cellular Data Network > MMS
APN: mms.gci
MMSC: http://mmsc.gci.csky.us:6672/
MMS Proxy: 209.4.229.92:9201

Cellular Data Settings (Same tab in preferences):
APN: web.gci

All other fields should be left blank. At this time 3G data should be disabled.

If you need tethering or other profiles they can be downloaded at:
http://help.benm.at/help.php