These articles are pretty interesting.
Studies on in-game advertisment
http://www.gamesindustry.biz/press_release.php?aid=13408 Follow ups on the unauthorized Subway advertisment in Counterstrike
http://arstechnica.com/articles/culture/subway-cs.ars/2 EA taking it to the next level
http://arstechnica.com/news.ars/post/20060221-6226.html