The adverts will run for 30 seconds from the 23rd of September until the 3rd of October, displaying a count of the number of players currently online in the game.
This kind of thing sounds rather interesting, though you have to wonder if it'll mean anything. A live count of people playing might get people to buy it but I kind of doubt it's going to push a whole lot of people. It's not the number of people playing that's going to get me to buy it, it's the number of my friends that are playing that would most affect my decision. What would be cool for the future is an Xbox live or Facebook ad that shows how many of your friends are playing or maybe even some sort of live streaming of one af a friend playing.
Anyway, Gears of War 3 is a pretty awesome game in it's own right so it deserves any masses of sales that this whole campaign will generate.