User blog comment:Arbington/ME3 Theories/@comment-1221773-20100717023925

I think it'll be sort of prologue-y stuff. Let's say there's a civil war going on in ME3 between the Systems Alliance and some group of anti-Alliance human colonies. One of the ME2 DLC packs could highlight an event that leads up to that war. People who never played ME2 or got the DLC will still be able to play ME3 and be given some background on the situation, but those of us who played the DLC got an in-depth eye-witness perspective of the trigger event.

The best comparison would be if BioWare had made each of the psychological profile events (War Hero: Skyllian Blitz, Sole Survivor: Akuze, Ruthless: Battle of Torfan) into standalone missions and released those before Mass Effect. They aren't necessary, but would've been cool to experience.