I noticed they weren't in the faction list for 4.0 or patch 4.1...
There's a lot of Italian city-states missing it seems. Urbino, Mantua, Ferrara, etc.
Nevermind, it may not have been on the list in the post, but it is in-game. Still, many city-states, duchies, republics, etc. should be playable factions.
Edit: Please no double postings ~ Ned