I'm not sure we would get Sonic announced publicly. Unless, of course, the E4All Nintendo announcement rumor is true, were Sega fans will be very happy.
Conspiracy Theory:
I think that the only reason they haven't announced Sonic is to not steal thunder from Mario & Sonic at the Olympics. That game needs as much hype as possible, whereas Brawl is a guaranteed Blockbuster. True, it will be the first time Sonic & Mario compete, but only because it is releasing before Smah Bros.
It was the trade-off for Sonic in the Brawl. Sega would be able to produce their own original game starring both Sonic and Mario for the Wii, in the hopes of capitalizing on the popularity of the gaming icons.
In return, Sonic was allowed to be a fighter in Brawl. Though he would not be the main focus, since he is one in many fighters in the game, his inclusion would make millions of Smash and Sega fans overjoyed.