Three new trademark fillings by Bethesda parent company ZeniMax Media appear to suggest that more new Fallout: New Vegas DLC is on the way.
One of the applications related to an in-game graffiti file like just the one that hinted towards the games Dead Money DLC last year.
The trademark's for the phrase "Honest Hearts". Also in the application were the phrases "Lonesome Road" and "Old World Blues".
Of course, the names could just mark possible routes Bethesda is considering at the moment, or they could be linked to something more concrete. Either way, that's all the info we have on new DLC on top of Dead Money at the moment.
There's good reason to expect more though. Bethesda has made no secret of the fact that it intended to prolong the shelf-life of Fallout: New Vegas with DLC.