This worked for me as well(reflashing without restoring from backup). Using new Apple Wireless keyboard.

There were a few quirks still. Here is the process I followed:
- Reflash without restoring the backup.
- Control Panel --> Bluetooth Keyboard --> (Generic 105/US ENG) Pair
- Gave me the option to enter the passcode, but would not take.
- canceled out to control panel and then reentered Bluetooth Keyboard
- Same settings, this time when I hit pair the screen jumped back to the panel to select what to pair with. I would click the keyboard and then click pair and it would go back to the available BT devices panel again.
- canceled out back to control panel
- went into bluetooth under connectivity
- got a list of devices selected the apple keyboard and entered the passcode hit enter
- failed several times
- got angry at the next passcode attempt and pushed about 30 keys in a random sequence
- paired successfully
- power cycled n800 and BT keyboard
- connected wonderfully again......fingers crossed this continues.