BioWare has revealed a full set of the Mass Effect Trilogy's content, including all the DLC associated with the collection. If you've been holding out for this sort of thing to get the full experience, however, you're going to miss out.
Here's the official list:
On PC, Mass Effect will include Bring Down the Sky and Pinnacle Station on disk. For Mass Effect 2, Cerberus Network will be included which features Zaeed – The Price of Revenge, The Firewalker Pack, Cerberus Assault Gear, Arc Projector heavy weapon, and Normandy Crash site mission. For Mass Effect 3, Online Pass will be included granting players access to co-op multiplayer.
On Xbox 360, Bring Down the Sky and Pinnacle Station are not included with Mass Effect, however they are available as stand-alone downloads through Xbox LIVE. For Mass Effect 2, Cerberus Network will be included and Online Pass will be included for Mass Effect 3.
Information on PlayStation 3 DLC will be available soon.
So, for the 360 version, you get Mass Effect, Mass Effect 2 and the Cerberus Network, and Mass Effect 3's online pass stuff. Basically, the 360 version gets the most vanilla of vanilla for Mass Effect.
The PC version is a bit better equipped, with the Bring Down the Sky and Pinnacle Station DLC for Mass Effect, and a bunch of in-game items for Mass Effect 2, with Zaeed's story and the very brief Normandy Crash Site mission. This does mean that there is no Lair of the Shadow Broker, no Arrival, and no Kasumi mission.
Don't worry, though, because the content is still available to purchase on the appropriately-linked stores for DLC. The team behind this was in no way obligated to add the content, but it also doesn't give much reason for current owners of the series to spend any money, nor does it entice people who want a full collection to do so either.
No word on the PS3's content so far, however. Maybe the PS3 Mass Effect Trilogy is the definitive version? What? Why are you laughing?