You have an option in Settings->Wireless & Networks->Mobile Networks->Network Mode->GSM Only (meaning 2G only). It's a basic Android setting, part of every ROM.
Also you could use the notification app for Toggle2G, it has a widget witch toggles 2G/3G.
By the way, the Automatic mode does not work correctly, at least for me, in RLS9, it did worked in RLS7. When I enable mobile data, it doesn't swich to 3G, but remains in 2G. I didn't wipe so it could be the problem. I'm OK with the manual method anyway.
Edited by elrond_bs, 23 January 2012 - 07:29 PM.