I haven't yet looked into the N900 wireless specifics or the aircrack package for the N900, but this would be the first place to start:

http://www.aircrack-ng.org/doku.php?id=simple_wep_crack

It isn't N900 specific, but should point you in the right direction. You might want to try this on a laptop using Backtrack first so you can familiarize yourself with the process and adjust it (if necessary) for the N900.