Randy Pitchford's beef with Valve is basically this:
1) Valve is going to release, let's say, Half-Left 4 Dead: Portal to the Team Fortress Strike, in 2010.
2) Gearbox is going to release Borderlands 2: Even More Boring, six months before that. Since it has a PC port, Take Two has decided to put Even More Boring on Steam.
3) So EMB sells gangbusters on Steam, netting a good profit for Take Two, and Valve takes a cut.
4) Pitchford argues that the money from Borderlands 2 is going to go in to the development or marketing budgets for their next ultra-game six months down the line. So when Gearbox releases Borderlands 2 DLC, probably about chimps flinging shit at each other, around the same time Valve is releasing their game, Valve gets a leg up off Pitchford's work with Borderlands 2.