Here's Gamespot's story on it here, they do a better job telling things then I could. But now for my opinion, I think it's in bad taste for sure to have people play as terrorists, mainly because allot of people aren't going to want to do that. So in a game where you've largely killed terrorists before, for them to stick it in is a surprise and not a good one.
Me myself I have no problem playing that part of the game because I know it's a game. To me it's just as if I read it in a book or watched it in a movie. Still I see this doing nothing but hurting Modern Warfare 2. There will probably be allot of people who will not buy it just because of this. And in this sue happy days I would not be surprised if some distraught family who loved ones died from terrorists don't sue them at some point.
One things for sure is this will get them publicity, sadly for a game that's so well known it's hardly needed.
There also going to have some multiplayer modes play in third person insted of first. Which is fine as long as theyt don't mess with team death, normal or hardcore.