In addition to pushing out the tools necessary to muck with the game, 2K has set up a detailed wiki about mods and modding and has also busted out some information on what can be done in this latest Civ as compared to others. For example, you can now combine mods with a cute "enable" button that scans to see if two or more mods can be mixed together. This wasn't possible in Civilization IV, so expect to have an easier time mixing and matching mods this go-around.
As you'll notice in the wiki text, there's one big thing missing from this release: the ability to use mods online. "Multiplayer and modding is something we wanted to take extra time on in order to ensure a stable and fun experience," 2K has written in the wiki.
More support will be coming in the future. 2K has said that it is aware that some stuff isn't working as well as it could, while other things aren't as "extensible" as they could be. Upcoming patches will, however, address all issues.
Don't know what Civ 5 is all about? Check out this Quick Look: