I'm just going to mostly copy-paste my SA thread about this...
In-game ads. To be honest, I haven't seen enough big examples of it to form an opinion of it - Battlefield 2142 was using placeholder art for them when I played the demo, and while there's some other known examples, I either haven't played them or forget their names right now.
Apparently Valve plans to test in-game ads out on Counter-Strike 1.6 in the near-future. The website CS Nation
got a hold of Doug Lombardi and asked questions about it. Not everything was answered, but it's an interesting read.
Interesting... stash your deagles, people.