What guilds are in Skyrim?

Bethesda released information regarding the return of the Dark Brotherhood in Skyrim. That leaves me curious about whether or not specific guilds will be included in Skyrim as well. I'm most curious about the Thieves' guild but would like information about all of them.

The Cyrodillic Fighter's Guild from Oblivion does not return in Skyrim. Instead, you may want to check out The Companions, a group of soldiers, fighters, and mercenaries for hire.

Likewise, the College of Winterhold is the replacement organization for Cyrodill's Mage's guild.

The Thieves Guild is only a myth, of couse. But if it was real, I'd lurk around the city of Riften to find it.

Information is a little sparse since the game has yet to release in most areas, but all 4 of the main guilds are making a return in one incarnation or another.

In addition to the Big Four already mentioned, SkyrimNexus reports (in the "Factions" section at the very bottom of the page) that "Bethesda Softworks has also confirmed that the Imperial Legion is joinable."

Confirming this, one of the three-hour preview reports I've read talks about encountering a Legion encampment and being offered a chance to join after helping the soldiers repel an attack. (He doesn't say whether he took them up on it... but he did steal one of their horses, so I assume he turned the offer down.)