BioWare and EA will be providing an early taste of Mass Effect 3 via a pre-launch demo early next year. Due out in January — no exact date yet — the sampler will offer both single- and multiplayer content.
For access to the multiplayer, you’ll have to have an activated Battlefield 3 online pass (for any platform). Those who do this will get early access, too. By how much remains to be clarified though.
There will be another way to access the multiplayer, too, but BioWare remains tight-lipped for now.
BioWare has set up a quick Q&A on its forums here, if you want it straight from the horse’s mouth.