Sonic HQ reports finding news of a fifth team from several sources, including the official PlayStation Magazine. PSM also included the names (which seem all to fake) Spunkie, Bunkie, and Ted. "Team Radical," as it is being called, consists of three wannabe heroes. Director Takashi Iizuka recently had an interview (Jan issue) with Nintendo Power. Iizuka-san does say that "As you play through the game, you will find many surprises and secrets on the way." Hmmm...three fresh, new characters...Sonic Heroes really is becoming the game I hoped for.
A little FYI: In the same interview, Iizuka-san says the game was in production for 20 months. That's a long time, damn it.
EDIT: Hmmm...other sources say it's in fact "Super," not "Radical."
EDIT NUMERO DOS: *Sniffle sniffle* Funk11 on the IGN boards...got the strategy guide from EB...no friggin' unlockable team! AAAAGH! But he also revealed that Sonic turns super to defeat the final boss, Metal Madness.