PhoneBoy provides us a tip about something I’ve been doing since day 1 of owning my Nokia E61.
Hit the Application Key on your Nseries [any S60 device really] phone and go to Tools > Settings > Network > Network Mode. Set it to GSM. You will be asked to reboot the phone for this change to take effect. Once the reboot occurs, the phone won’t try and use UMTS at all.
No sense in leaving it on if it does nothing but suck power! The same thing can be said about Bluetooth, turn it on only when you need it.