"not permitted to publicly link a UDID to a user account"? Really? But that's exactly what Plus+ is doing:
Register in Plus+ game 1, then open Plus+ game 2 and it promptly recognizes your account.
Appalling.
Register in Plus+ game 1, then open Plus+ game 2 and it promptly recognizes your account.
Appalling.
But it also says "Never store user information based solely on the UDID. Always use a combination of UDID and application-specific user ID."