And I think still Bluetooth uses less energy even with these new powersave-modes in Wifi.

I nowadays have Bluetooth networking on always when at home and much of the maintaining and configuring I do over ssh from PC.
It does not take much battery having it connected via BT all the time, but I would guess (have not tried) doing same with WLAN would drain the battery.

I used to keep it connected through USB, but I am not happy N900's CPU running constantly at 500 Mhz when connected via USB. I hope there will be a hack in kernel-power for that some day.