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It seems that Mary-Jo Foley wouldn’t rule this out and she does bring interesting arguements to the table:

• Exchange 2007 will not be supported on Windows Server 2008 R2 (now available to business customers with software assurance)
• The Exchange team has said to expect the product to RTM before the end of 2009
• More recently, company officials said to expect Microsoft to “launch” Windows Server 2008 R2, Windows 7 and Exchange 2010 together

As usual when Mary asked a Microsoft spokesperson she was told:

“We have said that Exchange 2010 will become available in the second half of 2009. There’s nothing additional to share at this point.”

Source: here