All 3g phones I've used to date, including three Nokias, will use 3G mode in preference to 2G (if they are configured to use 3G when available).

Unfortunately they do this even if the 3G signal is very weak - too weak for usable voice calls and useless for data.

So you have to disable 3G so that you get a usable 2G signal. Then voice is usable, and data is faster.

I've also noticed that in places where the signal is borderline, that sucks the battery dry very quickly when it's repeatedly trying to establish a 3G connection then drops it. As in, battery lasts about 1/3 of the time, compared with being in an area with stronger signal.

Those have been my experiences, anyway.

Perhaps related: All S60 phones have a setting "use packet data connection when needed" instead of "when available". They seem to come with "when available" out of the box (at least when supplied from 3 UK). Manually changing that to "when needed" saves a significant amount of battery life on S60 phones, and I've noticed no side effects.

Is there an equivalent to that on the N900? Sure, you need packet data enabled if you have even just an IM account online (due to silly network design, but I digress...). But those are the sort of things you'd turn off to maximise battery life when going on a long trip with no power available.