For those Nexus One users, let me know if this works for you.  Found this on a comment in the TmoNews forum, looks like it should work:

“Download the anycut app, create a new shortcut, actions activity, phone info, go to the shortcut on the home screen, change from WCDMA preferred to WCDMA only this will lock the phone to 3g only…leave a 3g area and you will have to change it back, inconvenient but functional.”

I’m keeping the Phone Info shortcut on my G1.  It has a lot of cool info even if I don’t need to fix the 3G problem.