Not sure if it will effect the forum market but it is a good thing for bitcoin that it has been accepted at Steam because now more people will notice it.
So Bitcoin has been making so good progress with these deal with different companies.
Yes its good for the bitcoin and its again a big company that is accepting it right now, its by the way also making it more popular so that is another nice thing.
On the other hand I think the forum market will not chance because the prices here are way cheaper than on steam itself!