As of today, Civ V fans can join the ranks of TF2, Skyrim, Dungeons of Dredmor, Dota 2 and Portal 2 players in browsing, uploading, and subscribing to mods through Steam Workshop.
Among the 144 files already uploaded is a re-creation of Westeros, the world of A Song of Ice and Fire and Game of Thrones. All that's missing is for someone to mod in the major houses and unique tech trees accurate to the setting. So... someone get on that. I command thee, Civ community!
If you haven't read it already, also check out this preview of the Gods and Kings expansion , a mere five days from release.