Steam offers gamers in-game downloadable content (DLC) with claim that it doesn't matter where the original game was purchased!

See the official Steam news article...

On 16 Mar 2009, Valve announced that they will be offering DLC for games through Steam; beginning with "The Maw" (which looks like a really fun Indie game; I highly recommend you take a look at it.)

The most important paragraph in this announcement is:
"DLC can now be added to any game on Steam, regardless of whether it was originally purchased via Steam, at retail, or via other digital outlets. It is also a feature of Steamworks, the suite of free tools and services available to game developers and publishers."

While nothing official has been announced, I hope this means we will soon see the Mirror's Edge DLC as well as a changed Ubisoft that will decide to release the Prince of Persia: Epilogue via Steam.

Thanks Valve! You continue to rock hard!