"The entire mission set is said to be a bit longer than the previous releases, and unlike the old ending, finishing the new content in Broken Steel doesn't roll credits and force you out to the main menu. You're now free to explore the world as much as you like. With additional experience levels and perks to earn, it sounds like you'll have plenty to do."
Well, that's enough for me to go after this expansion. I just finished the game, and I was thinking about completing the other sub-quests, when I saw that was not possible.