It actually worked, just once, from my laptop to the n900. Then it stopped working some minutes later, while I was testing from the other end.

Running 'tcpdump -i wlan0 -n multicast' on both ends shows requests being sent, but neither end receives the requests from the other.

I am beginning to suspect the router (which has really b0rked firmware, e.g. local dns lookups return the router's IP, even for non-existent addresses!).

Or possibly the radio in my n900 is dying, because it kept not being able to see the wlan.