If you've already played the game and its expansion, you'll still have reason to check out the remake. McMillen promises it will feature another Wrath-sized expansion, a new final chapter and ending, two more playable characters, and a ton of other goodies. It will also bring some cut content from the Flash version, including 2-player local co-op, a 16-bit styled graphical demake, and some new music and remixes.
According to a blog post by McMillen, Cave Story's Nicalis is assisting with development of the remake. While a PlayStation 3, Vita, and Steam release seems certain, Nicalis is also talking with Microsoft and Nintendo, although Nintendo has previously rejected the game. McMillen also says they're "looking into" an iOS version.
As for timing, you'll have to wait a while. The game isn't going into full development until next year, and he's only tentatively targeting the end of the year.