Well I suppose it'll have a good effect regardless of the motive
How can they get accurate stats on the PC versions players what with everyone running a dedicated serv.... oh wait...
There is a certain cynicism in me that says its less a marketing stunt than it is an attempt to put a warm and fuzzy spin on their rather tarnished image, kind of like a knitted cosy on a morning star ball

Why not just give them a large donation, without the player count? Telling everyone to hop on Live to get them to donate more is a stupid marketing ploy.

Yep, it would perhaps go a longer way if they kept it quiet and just donated 10% of gross sales to the charity. That way if/when the story broke months down the line about thier donation it would get them better PR, and be seen with less cynicism.
aliasalpha: that image of a knitted cosy on a morning star ball made me spit coffee all over my computer.....wow i need sleep

Well this is unfortunately the way these things seem to work. Children in Need for example. People arent just asked to give to charity. They are provided with expensive TV entertainment... sorry 'entertainment' and a bunch of celebrity people putting on sombre faces and flying off to go and be filmed with skinny black children.
Because otherwise most people probably wouldnt bother if they didnt get anything out of it for themselves. Same for Activision.

Probably because there is a business trick behind that and probably because they are not as charitable as they say. Blizzard did the same not so long ago with their pets "Pandaren Monk".

There's no publicity in that. Also, it doesn't inspire potential customers to think, "wow, if I buy that game and then play on Live, I'll help contribute to a charity!" and thus lead such people to buy the game under the impression that they've done their part in making a big corporation shell out to a charity.
Of course, such a contribution is also going to be tax deductible so it's really great for Activision. They get a tax break and boost sales while looking really good to some.

Good ol' Bobby is still the CEO, therefore I am inclined to agree.

Cyanical game marketing hype.. hehehehe..

Children in Need encourages people to get off their arses and raise funds through self-developed events, rather than just give money. There's less involvement with asking people to pay $60 for a game, $15+ per month internet access, then just sit down and play an over-hyped crap game.
I know which I'd rather do (and used to do until current health conditions put stop to it :-\ )

Wow, you guys really don't like MW2 :p
On the topic of making it a promo: Yeah, they could just not say anything and give money. They probably actually do that rather often (many companies do, if only for tax purposes). But by making a big deal out of it, it builds awareness. And then, when those gamers decide they want to be charitable, maybe they'll consider donating some money.
But nah, everyone should wear a hair shirt and cut themselves when donating to charity. Because that is the best way to build awareness of a cause :p