I think the only reason they changed the US date to TBD was in case something bad happened. Sakurai mentioned completing the game without incident, almost as if something had already happened. They seem to be localizing the game in both english and japanese at once, maybe something came up with the japanese localization and it had to be delayed, and he's not sure if that'll affect the US release yet?
Regardless, if it means that the roster will be another character bigger, I don't mind just a month...
But, yes, I do feel like both revealing Sonic, and the pushing up of the Mario & Sonic release, was to compensate for the fact that the game might get delayed.
This sounds bad, but maybe they want to delay it until january just so they don't have to compete with Halo 3 for game of the year. IMO, Brawl will deserve it more, 'cause I heard Halo 3 was trash compared to Halo 2.
However, Brawl coming out in december really doesn't make it deserving of "game of the year". That's as goofy as Twilight Princess being named "Wii game of the year" in 2006. Oh, wait, IGN did that...
But Halo fanboys are friggin' rabid and maybe Sakurai is sure that Brawl will be better, and it will gain more attention if released at the start of a new year.