According to IGN, Bethesda has announced that the first Fallout: New Vegas add-on will be released already during this Christmas season. However, it will unfortunately be exclusive to the Xbox 360 platform. Like with the Fallout 3 ones, it's most likely just a timed exclusive, though.
Quoted in the article is Bethesda's own Pete Hines:
We're excited to continue the partnership between Bethesda and Microsoft, and build on the success of the game add-ons released for Fallout 3 on Xbox Live. (...) Fans will once again be able to continue their experience in the Fallout universe with the add-on packs planned for after the launch of the game.”