Anyone who pre-orders Mass Effect 3 through the publisher's digital distribution platform will get a free copy of Battlefield 3. And even though we expressly mentioned Origin in the first paragraph, we'll say it again: this is a PC promotion only. Don't go thinking EA is going to mail you a PS3 copy of Battlefield 3 or anything.
Interested parties can head over to the Origin site. Everyone else, you can safely go back to playing the Mass Effect 3 demo.