The SmartReflex power saving feature of the OMAP processor is disabled by
default on the n900. To enable it:
You can also enable it by editing /etc/pmconfig and changing the appropriate
values to 1, and rebooting.

I've been logged in to my wifi accounts for 10+ hours now, with GPS on, and the
battery is only down by about 1/3.

Some power saving can also be achieved by setting your status to be offline and
disabling networking.

For further information follow the bug https://bugs.maemo.org/show_bug.cgi?id=6615 which has a somewhat higher signal/noise than this thread...