I don't know if I have an "ultimate" ipcc, but I have everything working on my 3GS running 3.1.
If you are on a non-iPhone messaging plan, and AT&T has provisioned you for MMS but it still isn't working, go to Settings>Network>Cellular and make sure the fields are as follows:
APN: wap.cingular
Username:
wap@cingulargprs.com
Password which is case sensitive: CINGULAR1
MMSC: mmsc.cingular.com
MMS Proxy: 66.209.11.32
I've found as I updated to different 3.0 betas and 3.1, that often the username and password would disappear despite loading the same ipcc. Though, on the flip, MMS has sometimes worked when they were blank. But, if you fill them out all should be right with the MMS world.