ESRB. Sonic Adventure was recently classified by the Korean ratings board, and briefly appeared in January in tandem with an "extend package," purportedly seen on the Xbox marketplace.
Sonic's companion on the great Sega ratings board journey, Crazy Taxi, has apparently not yet been re-classified for current consoles by the ESRB. We've followed up with Sega for details on both titles, but with E3 next week, the publisher likely wants it to be a "reveal" for the show. Start practicing your "we had no idea" face for the surprise next week.