When I read that FBI agents were conducting last minute interviews with neighbors of Plame I thought, Mmmmm, he's going to ask for an extension, these aren't LAST MINUTE kind of actions. Why? Because the investigation is moving on the the real meat - Miller, Plame, Wilson and the CIA. THAT will take time.
What no one seems to realize is that if he wants to pursue additional avenues of inquiry, but feels that he's already presented enough evidence to indict certain targets, then what he would do is get the indictments he's already established and then impanel a new grand jury to continue the investigation.
That makes more sense than anything else at this point. I guess we'll have to wait until the leak investigators leak more information to us.