Wireless keyboard won't connect

Apple wireless keyboard (2009)

Mac Pro 5,1

Lion 10.7.5

Apple wireless keyboard won't connect after changing batteries. It is listed as a paired device under Bluetooth preferences but using the 'Connect' option fails. The keyboard does not go to discovery mode.

I'm answering my own question because there seems to be a lot of voodoo around the problem. The following worked for me after trying a lot of different stuff.

1. Go to System Preferences -> Bluetooth. Delete the keyboard from the list of devices by pressing the '-' button.
   Alternatively you can delete the plist file but it shouldn't be necessary if the keyboard is the only problem. Deleting this file means restarting all other bluetooth devices to force them to reconnect. The plist file is in /Library/Preferences and is called com.apple.Bluetooth.plist.
2. Take the batteries out of the keyboard and reinsert them - don't power on the keyboard yet. This puts the keyboard in discovery mode the next time it's switched on.
3. In Bluetooth Preferences press the '+' button to add a new device.
4. When the Bluetooth Setup Assistant is displayed hold down the power button on the keyboard and don't release it! The keyboard should now appear in the list of devices.
5. Press 'Continue' - remember to keep pressing the keyboard power button while hitting 'Continue'. You should get an 8 digit passcode from the setup assistant.
6. Now release the keyboard power button and enter the passcode.
7. Press return - the keyboard is now paired.
8. Go to back to Bluetooth preferences and select the keyboard in the device list.
9. Press the gear icon and select 'Update Services'. Without this last step you may have to reconnect the keyboard from the Bluetooth menu after every startup.

Hope that helps.