There is yet another way...
From the dialer, type exactly:
After a moment, a screen will appear called "Testing". Select the drop-down that says "Phone Information". From there, scroll down to where it should say "WCDMA preferred", and change it to "WCDMA only". Use the back key to get out. It should stay selected even through a reboot.
This works on CRC1 (the latest US 1.5 "Cupcake" ROM) but I can't say for sure if it will work for other phones (myTouch) or other markets (UK).
It's essentially the same as AnyCut/BetterCut, you're just using a different way to get there. This uses more battery than 2G (GSM), but everyone already knew that, right?