According to a PCWorld article:
With Vista just out the door, Microsoft Corp. is now drawing up plans to deliver its follow-up client operating system by the end of 2009, according to the executive in charge of building the product’s core components.
It looks like they also have a name for it besides “Vienna”:
Last year, Microsoft said that the code name for this Vista follow-up is Vienna, but Fathi said he could not disclose the current name. “We’ve been told not to use it publicly,” he said.