That sounds familiar. Like I've read something like that before.
Ahh, perhaps I have!

N900 - WiFi Network Connection Error?
n900 wifi problem (<-- start here and follow anthonie's advice)
My n900 cant find any available WLAN
[N900] issues with wlan (now after reboot device warm/poor battery)
problems with wifi
Nokia N900 Wifi Issues
Unable to connect to wireless
Probably fried Wi-Fi chip - possible to diagnose?
N900 Wifi is dead
Wifi module in pain

The troubleshooting advice seems to always follow the same pattern: ifconfig, dmesg, lsmod, modprobe.

I would also try,