2K Games confirms this morning that Borderlands 2, which arrives in North America on September 18, will be powered by Steamworks on PCs. What this means is that even if you pick up a retail copy of the game, your play time will be hooked in with Steam's services and community features.
Valve launched Steamworks several years ago. It's essentially the Steam version of what Microsoft's Games for Windows Live is. For the purposes of Borderlands 2, playing on PC means you'll unlock Achievements, receive DLC and updates and link up with other online players through Steam. You'll also be able to make use of the Steam Cloud service to store your game save remotely, and access it anywhere.