I've managed to get my cellular phone (a Sony-Ericsson z520) working as a remote input device, but in order to get it working, I had to:
Open Bluetooth preferences
Select the service tab
Select the input service
Click the add button under the list of input devices
Click on the "show all devices checkbox"
Select my phone to be added
Once this is done, I can successfully initiate a remote control session from the phone's UI. If I remove my phone from the list of input devices, then attempt to initiate a session, the phone says it is connecting, then that it's "connecting remote control", and then it fails there. It would be better if this worked without needing to add the phone to a list of input devices before making an attempt.
Triage recommendation: Wishlist