Hmm, well I don't see any reason for them NOT to give CJ at least a minor role in the game, it would be amazing if they did, and the man seen in the trailer did bear a slightly similar resemblance to him, though not enough to fully convince me, although at that notion he could also just be one of CJ's crew.
The homeless man as Niko is pretty funny, but it wouldn't be likely for him to appear in the new game homeless and that much out of shape already. Even though it does seem like something Rockstar would do, slip their past main characters right under our noses in the trailer.
And in the middle, no way that's Tommy, he would be way older than the man in the trailer by now given the time frame. And from what I read in GameSpy's trailer impressions at this link, http://pc.gamespy.com/pc/gta-next/1211355p1.html, it is very likely that man is a ten years older Claude from GTA 3, the accent really doesn't sound very Hispanic, and even if it was we never heard Claude speak anyway so for all we know he could have a Hispanic accent. And if you close your eyes while listening to the trailer and try to imagine an aged version of Claude and what he would sound like, I think you may find that Rockstar found a voice actor that portrays him perfectly.