Valve's Doug Lombardi says that the company will be making its own television ad for Portal 2.
He says that the reason for this is because the advertising and creative agencies they approached came up with bad proposals to market the game, ranging from the clichéd, to those copies from the ad campaigns of other games "almost note for note." In some cases, the agencies didn't even pay attention to Valve's suggestions.
He believes that as the creators of the product, Valve is in the best position to make the ad. They are also able to make changes to the ad during production as it was playtested. With an agency, there wouldn't be enough time to perform such tests, with most feedback winding up as useless notes after the fact, instead of being put to use improving the ad.
Valve has long since professed its desire to make animated films, and this is especially evident in the shorts they made to promote Team Fortress 2's classes. It will be interesting to see what they do with the Portal 2 advertisement. With any luck, it'll be better than anything an ad agency could've done.