It's odd that so many of you guys are seeing this, we're not able to replicate the error. Has any of you tried reflashing the firmware?
I'll give that a shot today and manually configure everything again. I won't reload my current configuration.
I don't really use UPnP so I can't comment there, I just cannot keep any of my port forwarding and virtual server settings after the router reboots, but every single other setting stays.
UPDATE:
Well I opened up IE8 (Windows 7 RC), loaded the defaults on the router, then enabled Compatibility Mode in IE8. Then I logged back in and flashed the firmware again. Once it was done rebooting, logged in and restored defaults yet again. After it rebooted, then I went ahead and configured all my settings (didn't load them). Now all seems to be working fine.
Previously I had updated the firmware using Firefox 3.5 Beta 4, no idea if using IE8 in compatibility mode helped or not but might as well detail what I did precisely.
EDIT: Spoke too soon I guess. Just logged in and everything was gone...