I'll start with the basic answer that will diffuse the majority of questions here:
We've announced Street Fighter 4 to let everyone know it's in development. We aren't really providing any more information than that at this time, but over the coming months, you'll see bits and pieces come to light. Platforms especially aren't being discussed.
While CUSA owns the rights to Street Fighter, we have contracted Capcom Japan to handle its development (we felt they would, by far, do the best job with the title).
The game is further along in development than probably most of you think, but as we've stated, it's still quite a ways off. Again, the announcement was just to get it on everyone's radar, nothing more.
Beyond that no one at Capcom will speak about characters returning, character count, platforms, features, 2D or 3D or 2.5D, whether the trailer is representative of gameplay/character design/visual targets, arcade or not, online features, downloadable strategy, where it fits in the canon, etc.
All of that will be revealed over time... the next bits will be revealed a bit later this year.