For interested early adopters, 2K has released a beta patch for Civilization 5. This patch is not freely available at the moment, but needs some steps to go through on Steam. For the exact steps please see this thread.
There are no release notes available for this patch, but users report that e.g. a retire button and multiplayer animations have been added.
It is also possible to get early access to the Steam Workshop, which will replace the current modding database on GameSpy, and to new SDK beta tools, but you have to be a modder to get into this. It requires a manual verification via email.
For the exact procedure please see the same thread as mentioned above.