Go! Gaming Giant is another site that interviewed Jeff Gardiner about Mothership Zeta. Here's an excerpt:

How many quests are in Mothership Zeta?

It includes one large, branching quest.

We’ve heard Mothership Zeta is going to feature aliens. How exactly are aliens going to factor into the Fallout 3 universe?

Aliens have been hinted at, rumored and “Easter Egged” throughout the history of Fallout. In Fallout 3 seasoned explorers can find a crashed alien spaceship – in fact it’s there we decided to start our Mothership Zeta DLC.