Gamestop has begun selling the Valve Software catalogue on Impulse, its digital distribution service. The announcement might seem puzzling in light of the fact that Steam, Valve's own digital distribution platform, serves as Impulse's primary competition in the digital download arena. Beginning last week, the majority of Valve's library - Portal 2, Left 4 Dead, Left 4 Dead 2, Half-Life Complete and The Orange Box - has been available available on Impulse.
Just because the games are available elsewhere doesn't mean Valve has loosened its grip on its own software. Even if you buy a Valve game on Impulse, you'll still need to register and play it via Steam. You'll also need to play it through the Impulse client. In other words, you'll be playing Portal 2 on Steam on Impulse.
While this arrangement may seem counter-intuitive, since the companies are competitors, the reality is that any opportunity to sell more games is good for both parties. That said, I'm not sure why anyone would prefer to run two clients to play their game instead of one?