Well, looking at this image, you can see the silhouettes of a few different characters on the bottom.

This is merely speculation, but could the games multiplayer component be class based? From the image, there appears to be a heavy weapons-like type of guy, a sniper, a regular infantry guy (assault or whatever) and two others, maybe flamethrower or something.

I just kind of thought this, because they're having the multiplayer beta when ODST comes out, and wondered why they had those characters on the bottom, other than that it looks good. We don't know much about the campaign yet, so they could have put those characters as a sort of hint.

But, this is just an Idea, maybe it is or isn't class based, but I think it would be kind of interesting to see.

I guess we'll find out sometime in the next few months, thoughts?