This was posted in a comment over at TUAW, and may work as well.
"It's actually quite easy to turn EDGE off. If you can get access to the files on your phone (through any of the GUI tools available, or via TextEdit on the iPhone), do the following:
1. Open /var/root/Library/Preferences/SystemConfiguration/preferences.plist in your editor of choice.
2. Look for the string "apn". This will be followed by "wap.cingular".
3. Delete the "." in "wap.cingular".
4. Restart the phone.
5. When you try to connect via EDGE, it will fail.
6. To reinstate EDGE service, open the file again and insert the "." between "wap" and "cingular", then restart the phone.
I'm sure some enterprising person could write a native iPhone app to do this for you."