A picture is supposed to be worth a thousand words... and this one is taken from the front cover of Australian magazine PC Powerplay.
"That’s right – Mass Effect 3 is getting multiplayer! We travel to BioWare’s office in Edmonton for hands-on time with the series’ first foray into online gameplay, and chat with Mass Effect Series Producer and Mass Effect 3 Project Director Casey Hudson about the decision to go multiplayer in the epic battle for the galaxy."
So there. Unless the reveal is a big fat hoax - which is very unlikely - Mass Effect 3 multiplayer is definitely happening. We'll bring you more as we hear it.