The exclusivity complaint though....I really don't get that. That sounds more like whiny brand loyalty than a valid complaint. If Impulse had the numbers or the $$ to negotiate for those releases, they would have them. Complaining because Valve is more successful than Impulse and therefore you have to use their service to play X game....yeah. That sounds incredibly whiny. Calling it anti-trust sounds even more whiny.
IMO, I think it's more of a question about the Steam DRM than a Steam vs. Impulse issue. I only own 1 Steam-based game (Dawn of War II). I bought the boxed retail version and would have much preferred a version of the game that was not integrated into Steam. I only play single-player and could care less about achievements or other Steam "services." However, I still have to deal with Steam, even in off-line mode. God forbid if I ever wanted to sell the game or just give it away to a friend, something that Steam doesn't allow. So, my personal issue with Steam has nothing to do with its popularity, marketshare, etc. I'm glad that Valve has this distribution method and that so many people seem to like it. However, I guess I'm just somewhat old-school in my beliefs that a consumer deserves to be able to enjoy a product--or resell or give it away--without having to periodically seek the approval (or "authentication") of the distributor. But I digress.
If there is litigation, I would imagine that it would be in the wake of Steam's collapse, merger, bankruptcy, etc,, when customers actually read the Steam EULA and find out that there was no backup plan to "unlock" Steam-powered games. In that event, customers could possibly lose access to their games with no recourse but to file a court claim. There are many people who own hundreds or even thousands of dollars of Steam games in various parts of the world, so lawsuits are virtually guaranteed. And like any business would do when facing bankruptcy, failure, or costly litigation, Valve will act in the best interests of shareholders, creditors, and the bottom line, not in the interests of customers.