1. Fallout: New Vegas reentered the top 10 of all game sales in July according to the NPD report released in August for the US. That was primarily due to the across the board price drop. That the bump was that great had to have caught Bethesda's attention. I suspect that a lot of people who were otherwise interested in the game passed last fall because of early reviews that were negative based on technical issues (though those were largely exagerrated) and that the lower price and patches made the game a lot more attractive. So thats a bunch of new, recent players.
2. According to Steam, the #2 seller for them last week was Lonesome Road. As far as I know, none of the other DLCs have had that impact on Steam. If that kind of interest was duplicated on the PS3 and XB360, then its a huge success.
Both of those would indicate that there is stil a demand for FNV content. And as Chris Avellone noted in a recent interview, DLCs are low risk, meaning relatively inexpensive to make.
So I suspect there will be some renewed discussions at Bethesda over whether they are really done with New Vegas or not. Any new content DLC would be months away, even if Bethesda called Obsidian tomorrow and told them they'd pay for another to be made. But that might actually work in our favor, because while Bethesda is going to want to focus on Rage and Skyrim the next few months, taking a shot at another FNV DLC early next year wouldn't necessarily be all that risky or impact their other titles.
I'm not so positive about all of this to think its likely we will see more DLC, but I do think the odds have gone up.