It depends what you're asking for. Some people at Apolyton want a pure conversion of AC, or at least a very close conversion (takes place on Alpha Centauri, starts in the modern day, with 14 AC factions). This'll take a while, since it requires the SDK to code new rules for things like fungus, but it should make for a really distinctive game. On the other hand, SMAC never included concepts like National Wonders, religion, luxury and strategic resources, production overflow/storage, etc., so a true conversion might just feel a bit lacking after playing Civ4 for a while; you'd probably end up seeing something more like a "SMAC 2".
Others (like me) want to keep everything on Earth, keep the Civ4 rules and nations, and simply use the AC techs, quotes, movies, civics, icons, etc. to assemble a reasonably balanced set of "future" eras. In my case, I'm looking at 35-40 techs in three eras (let's call them Digital, Fusion, and Nanotech), with the spaceship win replaced by a Transcendence win at the end. This sort of thing shouldn't take nearly as long to make, assuming people start converting the high-tech unit graphics from civ3 mods over soon.