BioWare and EA will be releasing a demo for Mass Effect 3's single-player and multiplayer offerings come this January, and those who have a Battlefield 3 online pass will have early access to the multiplayer demo.
There's no exact date for either of the demos, and it's unknown how "early" of an early access the Battlefield 3 online pass will offer, though the single-player demo will be available to everybody on its day of release.
There will be another "program" where players will have a way of accessing the early access as well--presumably for non-Battlefield 3 owners--though BioWare didn't go any further than just mentioning it.
BioWare set up a FAQ on the matter, which also answers a few other notables about the demo.
