There is a poll at Firaxis where they are asking which game people want to see updated. SMAC is on top by a landslide. I've heard someone here say that Sid has said that he knows that a lot of people want to see SMAC 2, and that he is willing to consider it.
Personally, given the outstanding game that Civ 4 is... it would be criminal not to give SMAC the same treatment. To me, Civ is a great because it is about the development of human civilization, up to our time, but the game really only feels like a climax, not a total story. SMAC was the other half to the complete story. The story ends with a vision of where we could go from here.
Anyways, there are no plans in the works for SMAC2, but he knows that people want to see it.