I've had my N900 for a week (not using WLAN) and it have worked fine. Yesterday I tried to connect it to my WLAN router and immediately after it had connected it rebooted. It did that a couple of times (sw_rst as reason) until it did not boot anymore.

After reflashing the device it worked again nicely when it was not connected to my router but trying to connect always resulted in a reboot. Then I also searched for unsecured networks nearby and surprisingly there it worked without problems. No difference on my router if the network is secured or unsecured.

After this I did a hard reset on my router and even more surprisingly it seems that something in the router did caused the shutdowns as now everything seems to work fine....