During his keynote at the Microsoft Management Summit 2006, Bob Muglia, senior vice president of servers and tools at Microsoft, told the audience that the next major Windows server upgrade, code named Longhorn and informally referred to as the Vista server, will likely ship during the second half of 2007.
The company has said it would ship in 2007 but did not provide this guidance in the past.
The company is expected to release the next "near feature complete" beta in the very near future, Muglia said, and will release the next major server beta that incorporates all necessary Vista client changes this fall."
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I noticed when I launched Windows Longhorn Server build 5308 setup from within Windows XP Professional, a notification on the setup dialog said, upgrading from Vista Enterprise Server is not supported.