Worked for me 1st try!!!

I'm on a shared plan with my wife who has a LG with MMS. I did the .IPCC update (that enabled tethering automatically, but still no MMS), put my SIM in her phone and went to Media Net, put my SIM back in my iphone (never turned iphone off by the way), and BAM now have MMS. I can send AND receive with no problem! I have 3g with 3.0 firmware by the way. Just thought I'd contribute for those who are stuck...
If visual voicemail isn't working after this, just do this: Go to Settings -> General -> Reset, and tap Reset Network Settings. Phone will reboot and everything should now work fine.
If anyone's wondering, I followed these steps exactly:
How to Enable Carrier Bundle Support in iTunes 8.2 in Windows.
Step 1: Quit iTunes
Step 2: Press Windows Key + R to open the run dialog. Enter in cmd then click OK
Step 3: Enter the following command into the Command Prompt window.
"C

Program Files\iTunes\iTunes.exe" /setPrefInt carrier-testing 1
Step 4: Connect your iPhone and click to launch iTunes from your Start Menu.
Step 5: Select your iPhone from the list of devices on the left then hold down the Shift key and click the Check for Updates button.
Step 6: Now you can now select your .ipcc file as it is not greyed out
After that, I did the SIM swap part...