From Bioware's FB page:
"We can officially confirm that there will be a multiplayer feature in Mass Effect 3. However, please reserve your appraisal until we are able to give more information. Our Executive Producer Casey Hudson says it best: "Yes, co-op MP missions for ME3: they're real, and they're spectacular. Rest assured it's nothing of what you've feared. More soon..."
"Tune in for a special BioWareTV episode at 10AM on Wednesday October 12th to have all your questions answered! In the meantime, here's an adorable picture of Garrus:"
this could be interesing. Didn't Army of Two of Co-Op Career mode? and Gears of Wars?