I think there will certainly be support for various types of Organisations, with Guilds being just one of them. The Organisations Developer Journal is on the list of ones we are going to see in the future and I think that is going to give us a much better idea.
However, what we have at the moment is the impression of is a variety of Organisations including Guilds, Schools and something like Associations. We know a bit about Guilds and Schools from the Research and Technology Developer Journal. It looks like we have Guilds which get a benefit on the production of goods and can patent their research. School who get a benefit when researching, but can not patent anything they find. And some more social type of associations, I would love to see neighbourhood watch groups springing up for example along with those who go adventuring and are mercenaries for hire!