I'm gonna go back on mine now and say September 23, 2012 -- the day before Labor Day. What better of a weekend to launch?! Get it at midnight, and then spend all day Sunday and Monday playing the crap outta the thing. I also would like to change my price now to $349.99 -- with a pack in game that contains those goofy tech demos Battle/Chase Mii. If they're smart though, they'll pack in that New Super Mario Bros. Mii game. Would make a TON of sense, and would easily cause the thing to fly off store shelves.
I'd also like to suggest it comes packed in with a single Wii Remote, Nunchuk, and Classic Controller Pro, too. That way, NO EXCUSES for devs.