actually i think that it should affect price of bitcoin, but not by much, maybe by one or two dollars only as there are lots of people that use bitcoins and steam but that's still not enough
well, this is true that after steams accepting bitcoin bitcoin price will surely grow up . because lots of bitcoin users are using steam also. and lots of steam users like to pay in bitcoin because of it's easy payment system. but here is we are talking about the bitcointalk marketplace.
I don't think the official steam support of bitcoin will affect the market. because buying games from manual sellers are very easy and also cheaper then actual steam's price.