Of course DL1 but no chance Valve would've done this without a stern talk about where DL2 will get released. Or they're so, soooo much dumber than we ever could've imagined and indeed soft push-overs.
honestly, knowing volvo that crossover talk went something like this:
Techland: can we use L4D2 stuff in our game?
Volvo: yes
THE END
what i'm trying to say is - volvo lets pretty much anyone use any of their stuff ... as long as it's released on steam as well