I don't know what I did, but this has been happening for the last few weeks on my n900.
All I have to do is leave the thing on for a few hours and it starts sending out all these ARP packets and the battery dies within a few hours. The Internet connection doesn't work at all while this is happening, but forcing it to disconnect and reconnect fixes it for a while.
If I run tcpdump, the screen shows one of these per second:

(timestamp) CF +QoS ARP, Request who-has 192.168.1.1 tell 192.168.1.116, length 28

So, it looks like it's trying to find the mac address of the default gateway. Strange that it doesn't seem to get a response flooding the network... Anyone got any ideas on how to narrow this problem down? I don't want to start by restoring a backup or reflashing!

Only thing I can think of that messed with this type of setting is mad-developer, but I would think it's something else that is running in the background, not that.

install auto-disconnect: it automaticly disconnects you!
but maybe a real solution is to set a static ip adress settings->internet connections

well, i'm not sure if that will work. the connection isn't exactly idle, and i don't want to disconnect it anyways. I just want it to stop sending useless packets constantly. i will disable wlan and force it to use 3g data and see what it's doing in a few hours.