BioWare says Dragon Age II has sold more than one million copies in less than two weeks, and as a thank you note they are offering a free copy of Mass Effect 2 for PC to DAII owners.
Go here if you own Dragon Age II, just enter the code for the online pass that you got with the game (The Black Emporium DLC) and the rest should explain itself. Even easier, or if you had previously registered the code on BioWare Social but threw away the card with the code, just log in and click the "Lost my code" link to automatically activate it.
Activation should work regardless of the platform you bought the game on (it's a code after all), and apparently the free copy of Mass Effect 2 doesn't come with Cerberus Network access totally comes with Cerberus Network access and some DLC, haha!
[Thanks everyone who sent this in, and Max for checking activation protocol]