OXM Podcast has interviewed Todd Howard about the Fallout 3 DLC's...
[...]
"[The Pitt] is still scheduled for somewhere in late February?"
Howard: "It'll probably be early March-ish now."
"Early March-ish, and Broken Steel about a month after that then?"
Howard: "Yeah, they'll each be a month apart, maybe a little more, a little less, depending on where they are in development. We'd like to have something every 5-6 weeks."
[...]
more:
http://www.nma-fallout.com/forum/viewtopic.php?t=49194