This page is for the Gentouch78, and at the bottom there is a WiFi gadget that I use on my GT to turn wifi off when I turned airplane mode on. Maybe it will work on the N900?

Edit:
There's another app on there that I used to turn Airplane Mode on in the first place. I don't want it off (The simulated GSM radio still manages to drain the battery like crazy even though it's only a virtual radio. Yes.) so I haven't checked if it turns it off. But it might.