There have been blue posts on both the NA and EU official World of Warcraft forums both yesterday and today discussing how Ulduar will be handled on the PTR. Daelo and Vaneras both basically said the same thing, which is that Ulduar won't be constantly available on the PTR like Naxxramas was in the Wrath of the Lich King beta. It will be enabled only when Blizzard has something specific to test.

That being said, there's going to be a LOT of encounters to test in both 10 and 25 player modes. There will also be a whole lot of "hard modes" and achievements to test. We're shooting for most all of the encounters to have "hard modes" that allow raids to increase the challenge and reward level if they're up to it. We're pretty pleased with how Sartharion and his drakes worked, and we're going to expand on that idea extensively in Ulduar and future raids.

Looks like guilds won't have as much opportunity to completely learn the raid while it's on the PTR, although I'm sure many will use the testing windows to gain as much knowledge as they can on Ulduar.