That announcement doesn't actually say it will make the game playable after you beat the story. It says that it will expand players experience in the Fallout universe. It could very-well just be content that you play before the game ends.
That's how I think the DLCs will play out for New Vegas. *Maybe* add new alternate endings, but mostly just add locations and side-quests like they did with FO3 (i.e. all the DLC except for Broken Steel).
The problem with Broken Steel was that it raised the level cap in a way that you suddenly had access to 50% more skill points and perks. Which was over-balanced. If they had provided only 10-20% more skill points and 3-4 new perk choices, it would have let you round out your character slightly more and pickup a tiny handful of perks that you had missed out on. It was too easy in Broken Steel to have nearly every useful skill above 90 rather then being forced to trade-off.
Obsidian has done a good thing with the skill books and skill points rewarded at level up. The formula in FO:NV is (10 + INT/2) while FO:3 was (10 + INT). Skill books in FO:3 rewarded 1 point (or 2 points with Comprehension), in FO:NV there are fewer books. Each one gives 3 points (or 4 with Comprehension), which makes Comprehension a less overpowered perk. OTOH, Educated in FO:NV is still +3 points at each level up, which somewhat undoes the (10 + INT/2) change as it's like having an extra 6 points of INT in FO:NV.
But I'm only level 9, and on my first play, so I'm not convinced that it's good or bad yet.