Hello.

I can't see GSM only mode option in network settings? Anybody have an idea?

The phone came with;

V 3.0617.0.6
03-05-2006
Rm-92
Nokia N80 (54.02)
firmware.(T-Mobile/USA)

Because of that my battery dies quickly.
By the way Cash, thanks for sleep mode tip! It definetely gave me an extra time.

Maybe T-Mobile has required a firmware customization so that this setting is disabled for users?