You won't believe it! Someone leaked a picture of the upcoming L4D2 DLC "The Passing"! It looks like the first Survivors will accompany the second Survivors after the finale, making all the other stuff Valve said lies!
Try searching for GMod Mount 2, or go onto garrysmod.com and search through the news page. Downloading GMod Mount 2 will allow you to mount (therefore enabling use ingame) nearly all Source games, provided you own them.