I finally got this to work on my bros 16gb with rogers in canada.
After doing the activate mms through Cydia there was no "cellular data network" tab.

In the end i had to upload the carrier bundle to the phone through itunes ( iClarified - iPhone - How to Enable Carrier Bundle Support in iTunes 8.2 (Windows) ) , I had to use the Rogers 5.0 ipcc.

After uploading the new ipcc file to the iphone the "cellular data network" tab appeared with the correct settings for Rogers.
Now mms works fine , can send and receive MMS with no problems.

Found this video on youtube , click the more info to find the ipcc file for Rogers.
YouTube - iPhone 3.0 mms working rogers/at&t/ (iPhone BETA 3)