Oh man, I'm getting Potato Sack flashbacks! I'm assuming that, based on the Potato ARG, that the involved games'll likely all be loosely connected (They were all Indie, IIRC), but Ikaruga isn't Indie, and I doubt the biggest link between the games would be "Bullet Hell," right?
Then again, we knew about Portal 2 prior to the ARG starting up, and we knew the launch date was approaching, too. Unless Valve wants to one-up themselves and simultaneously reveal and release a game, I doubt this is going to lead to anything more than hats, sales, or something really, really minor (Maybe a DLC pack for another game? Or one of those rumored remaining TF2 shorts meant for a canceled [adult swim] show ala Expiration Date? I mean, that wouldn't be minor, but it's been awhile since we heard about them).