That's possible but I still think development time unusual.
Obsidian is after all a company that made it's position on sequels reusing assets and technology.
It also seem to clash with the rumors of free2play or iphones/XBL games which should take less time to develop.
They can't really re-use assets if they aren't working on a sequel with the same engine that the previous title was made with. I think MCA said in some interview they got games coming out of Onyx. Yes, in plural games. FNV was released about a year ago. At least some of the team went to work on the dlcs. If we presume the remaining staff went to work on New York after they were done with FNV that's about a year in development. That's if they started working on it immediately after FNV. Development time for AAA project can be anything between 18 months to 5 years. Though in Obsidian's case I would imagine 2 to 3 years is the maximum time they would get. So I wouldn't worry yet.
As for the XBLA game. Since it's supposedly indie, they won't announce it early on. It will be a lot closer to release date than 6 to 12 months, something like 1 to 3 months. They might even take advantage of the preview rounds for one of their other projects and show the XBLA game to the media at the same time (announce it earlier and then show it to selected media who come to see their bigger projects). Or that's at least how I would do it, with a very limited / non-existant marketing budget. And add some guerrilla marketing to that....
And who knows, how much focus they've had on the XBLA game, they might have had to shift people from it to work on the non-indie games to make deadlines.