After seeing how much difference 3G signal quality makes to some other S60 phones battery life (1 day vs. 2.5 days), and how much turning from "packet data always" to "packet data on demand" made (about 50% longer), I wouldn't be surprised if the 3G signal quality and settings are making a big difference to N900 users.

Hand-wavingly, that might even have some vague connection with why some people report better battery life with a new SIM - if it results in different 3G behaviour.