...I'd argue that them offering their DRM for free while other methods require money, could be construed as an abuse of their dominant position, as it forces other services to either discount (D2D, no one would be using them for Civ V if they were the same price), or not pick up the game...
You argue wouldn't hold up anywhere that mattered, unfortunately. For anti-competitive legislation to apply, Valve must be using it's dominant position to prevent other companies from making a profit. Companies, and services like Impulse, can sell Steamworks DRM-enabled games and make a profit. If Impulse sold Left 4 Dead, for example, Stardock would still make a profit. The problem comes with repeat customers. That customer now has both Steam and Impulse. Steam requires that it's in your face all the time
for you to play your game. Impulse doesn't even need to be installed on the machine it's are running on. Realistically speaking, with Steam's ability to offer massive discounts, repeat customers for Impulse would be less. This forces Impulse to canabalise their customer base in order to make a profit.
While this is obviously unethical, and with Valve moving into the Console Realm and brining Steamworks to the Playstation 3, their dominance is set to continue and expand. While Valve make some terrific games and have worked damn hard to make Steam the success it is, they are placing themselves into a position that many companies have been placed it.
Microsoft, for example, enjoyed a period where it was the underdog and all the 'cool kids' used Windows. Microsoft earnt it's customers respect and loyalty and published some industry leading stuff. Their level of success was unrivaled as a result, and it placed them into a position of dominance - they then abused that position.
Activision was founded by Game Developers who were tired of getting the shaft, and wanted a share of the profits of the games they made, instead of a simply salary. They published some incredible games and earnt their position as industry leader. They now abuse their position.
Absolute power corrupts absolutely. Valve is placing itself as the bridge between gamers and their games in such a way as to create their own, closed platform. This is unacceptable.