The press release from Twisted Pixel had some more info
This is the first of three DLC levels that Twisted Pixel Games has been working on ever since The Maw development wrapped last year. "There's a period of time where we're not allowed to touch the game because it's going through certification, but there was more we wanted to do, so rather than calling it a day we threw ourselves into making all new levels," explains CEO Michael Wilford. "We've never done DLC before so it's a bit of an experiment for us, but there seems to be a lot of excitement for it so I don't think we can go wrong."
Each DLC level will expand The Maw storyline by fitting in between the original levels as "deleted scenes". "As with any production, there are always pieces left on the cutting room floor that you wished could have made it into the final cut," says CCO and Director Josh Bear. "We had a lot of half-finished story elements and gameplay mechanics, so we are picking the best ones, polishing them up and making new levels that are bigger in size and scope than the ones in the main game."
The DLC will cost a whole 100 Microsoft points ($1.25), and at this point there is no news as to whether this will add any gamerscore points as well. Also attached are a few screenshots from the devs themselves.