The game is gone from both services in Europe because Ferrari: The Race Experience came out on PSN and apparently Ferarri's licensing deal means they get to remove the competition. It's still available for now on XBLA. If anyone is at all interested in picking this up, you may want to do so soon, because there's no telling how long it will be available.
Sega has announced the following:
“Due to the expiry of the contract with Ferrari, Outrun Online Arcade will remain offline on PSN but will still be available on XBLA until Dec 2011.”