I'm trying to download the OS from http://tablets-dev.nokia.com/nokia_N810.php. When I enter the WLAN MAC address I'm told "Product ID not valid. Please re-enter the 12-digit code."

I was actually able to get through once making me think there is an issue with their side but so far Nokia hasn't been much help on the Tech Support side of things. Am I the only one having problems? Does anyone have it in a place I can ftp it from?

I don't know, but it works for me.

Is this about an N810 or an N810 Wimax edition?

Try to use Nokia update wizard:
http://nds1.nokia.com/files/support/...ate_Wizard.exe
This wizard will download this for you when start update.