Sorry to bump this up, but I have exactly the same issue. Don't know if it's a problem with the N900 itself, it works perfectly with routers in all of my friend's houses.

I've tried adjusting all the settings, using it with WPA, WEP and without either.
I've tried a variety of DNS servers. I've tried switching the router to 802.11g only. I've tried turning off the router's own firewall.

All I get from the N900 is "network connection error", no details. I have a 500MB allowance per month, and i've used 230MB in just over a week, which I could cut by more than half if I could get this damn wifi working. It doesn't even show up on the router's DHCP client list, so I can't put it in a DMZ.

The router is a Belkin F5D8233-v3
Anyone have any ideas?

thanks..