Here's a package of Kismet for Maemo OS2008 on a N810. A N800 might work too, I'm not sure. You probably need to run it as root. If you don't have normal wireless after quitting, try iwconfig wlan0 mode managed
kismet_2009-06-R1+r2917-maemo1_armel.deb
Source is basically stock SVN kismet r2917 with an older debian/ directory. The same source should also build for Maemo 5, no idea if it works with N900 wireless though.