The last few posters need to get their heads out of their collected asses.
First off try a mod if you're fiening so bad for an update.
Legends of Revolution is a well polished processional quality mod, try it if you want an Expansion Pack experience. Also there are some other great mods that completely overhaul the game, providing a fresh new experience.
Rise of Mankind expands the tech tree and available units by like a factor of 5, and is perfect for anyone looking for a mega modpack experience and doesn't mind the hit to performance such added content demands; Hundreds of thousands of Civ4 players enjoy this RoM. Nearly a million players have tried
Fall from Heaven, firaxis messed up not selling this one actually, it's a great game in it's own right.
Planetfall is as good of an SMAC sequal as anything firaxis could have come up with; people asking for a new SMAC really need to try it. And there is
History of the Three Kingdoms for anyone interested in a fresh scenario based in feudal times. Basically anyone crying for a new civ update that hasn't checked out at least one of those mods is just wrong, and needs to get smacked upside the head.
As far as firaxis is concerned, the money is in streamlined MP based games like what they are looking into. Going for netbook and facebook applications is a smart business move, and could also be fun, further getting this working will help with innovation for the the upcoming Civ5 for the PC when it is finally developed. Right now though Civ5 isn't practical, for starters any new novel concepts need to firmly work in Multiplayer (wich the facebook civ construct will help with developing), and civ4 is still selling quite well, especially since it's sales are being partly driven by some great community produced content.
No, now's not the time for Civ5.