Atlus has been running an official site for Persona 4 The Ultimate in Mayonaka Arena since the game's announcement last year. Now it's Arc System Works' turn. Arc opened its own site for the arcade version of the game today. Access it here.
The site has information about the game's NESiCA IC card support and Players Guild features. Via the P4U Players Guild site, you can edit your profile, buy character colors and other goodies using the P$ (Persona Dollars) that you earn while playing the game, view your ranking, search for other players, and get the latest event information.
A Q&A at the site offers further details on the NESiCA and Players Guild features for those who can read Japanese.
Separate from Arc's site, Atlus updated the game's main official site with a tutorial video. You'll find it in the "special" section. We've also included a YouTube version here: