No way a science fiction game could use Civ. .
I think Firaxis also has some rights to the name Alpha Centauri since they developed it. So the rights are split between Firaxis and EA. I did a quick search on 'poly and saw somewhere (I can't find it now) where Brian Reynolds said BHG had no rights to SMAC.
Planetfall is a mod for Sid Meier's Civilization 4 that uses the storyline and characters from Sid Meier's Alpha Centauri, combined with gameplay that takes the best from SMAC, Civilization, and the modding community.
Planetfall will not be a clone of SMAC using the Civ4 engine, but rather will incorporate the Factions, general storyline and Characters combined with the newest gameplay modifications to allow for more strategic gameplay.