Lies. Ambiguous LIES! It would just make Nintendo look bad if they had exclusive content for American release only, especially with characters!
I dunno if this is pointed out already but...
The American NTSC and PAL versions of Kingdom Hearts 1 had boss fights with Ice Titan and Sephiroth for example. Japan got a re-release of Kingdom Hearts named Kingdom Hearts Final Mix later.
Same case with Final Fantasy X. The PAL version got the Dark Aeons as extra bosses, while Japan and America got nothing. (Well, the Final Fantasy X International release in Japan has them)
Maybe this doesn't have anything to do with Brawl directly, but it has happened to other games in the past, so I don't see why it can't happen to Brawl.
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Further, I hope this is true. We'll just have to wait until March to see if this is really true though.