-- The Boros Legion (red/white)
-- The Golgari Swarm (green/black)
-- The Selesnya Conclave (green/white)
-- House Dimir (blue/black)
-- The Orzhov Syndicate (white/black)
-- The Izzet League (blue/red)
-- The Gruul Clans (red/green)
-- Azorius Senate (white/blue)
-- The Cult of Rakdos (black/red)
-- The Simic Combine (green/blue)
I have been playing Golgari and Dimir and didn't even know it!!!
For Return to Ravnica these are the five guilds that will get cards:
--Izzet (Blue/Red)
--Selesnya (White/Green)
--Golgari (Green/Black)
--Rakdos (Red/Black)
--Azorious (White/Blue)
Here is the reference for my post on the five Shards of Alara.
The remaining guilds are the focus of the second RTR block set, Gatecrash:
-- Boros (Red/White)
-- Dimir (Blue/Black)
-- Orzhov (White/Black)
-- Gruul (Red/Green)
-- Simic (Green/Blue)
Here are some sample cards from the Return to Ravnica prerelease and a Ravnica Block junk common lot I just received (September 27, 2012). The white/black example is from a Alara Reborn common lot.
-- The Boros Legion (red/white)
-- The Golgari Swarm (green/black)
-- The Selesnya Conclave (green/white)
-- House Dimir (blue/black)
-- The Orzhov Syndicate (white/black)
-- The Izzet League (blue/red)
-- The Gruul Clans (red/green)
-- Azorius Senate (white/blue)
-- The Cult of Rakdos (black/red)
-- The Simic Combine (green/blue)