Well, if the trailer does indeed confirm that Ash is in BW (as in, he is seen interacting with the BW cast in some way), then I don't have any reason to doubt it anymore.
Seriously, though, it's not like they couldn't just leave an Ash appearance in the trailer for about a second, a small amount of time, and then keep the rest of the trailer focused on the new characters. I mean, by even allowing for him to appear for a second, it allows the new characters to get the focus that they deserve for pre-releases, and yet it confirms to everybody that Ash will be in BW. If they did that, then no one will even HAVE to question whether he is in BW.
And honestly, if new kids were to come in and Ash was removed, I doubt they'd even care. If they didn't even care if Misty, May, Max, Tracy, and Dawn were removed, why should they care if Ash is removed. As far as the new kids are concerned, he's just another character, and probably doesn't even exist, even IF he was the star of the anime. I mean, if Pokemon Special can remove even Red without any consequence, they can do the exact same with Ash. It's that simple, really.
Well, Ash is going to stay. I just hope that THIS time, they actually DEFINE the Pokemon Master goal absolutely.