A 2G connection is shown to be less power hungry than 3G. Since the N900 is an 'always on' data device, using 2G as its main source of data for low data intense applications (email/widgets) would make sense. However if you wanted to say surf the internet/stream music, 2G is just too slow.

My idea is:

An application that keeps the the default GSM connection as 2G that then switches to a 'heavy data mode' (3g/wifi) when specified applications are opened (eg internet browser, spotify, internet radio etc). This would increase battery life considerably.

http://maemo.org/community/brainstor...battery_saver/

It sounds fairly easy to create too..