There have been reports of this method working and other reports of it not working.
On my phone when I select GSM it reverts back to WCDMA preferred.
This is from this thread at backflipforums.com
You can turn off 3G by creating a shortcut with the app Any Cut. Here are the steps:
- Run Any Cut
- Select New Shortcut
- Select Activity
- Scroll down and select Phone Info
Tap the new shortcut and scroll down to:
Tap the box where you see WCDMA referred
Select GSM only
You can restore 3G by opening the shortcut again and selecting WCDMA preferred