On the Microsoft Exchange Team Blog, Scott Schnoll writes about Exchange Server 2007 in combination with Exchange Server 2008. Among other things, he writes that rolling cluster upgrades are not supported. This means that if you have existing Windows Server 2003 clusters, you need to build a new Windows Server 2008 failover cluster and move mailboxes with the Move Mailbox operation. This is not a limitation posed by Exchange. Rolling upgrades from Windows Server 2003 to 2008 are not supported in general (see this).
More details: You Had Me At EHLO... : Exchange Server and Windows Server 2008
Note: I already tried to install Exchange Server 2007 SP1 Beta 2 on Windows Server 2008 and the process failed during the configuration of the client access role when the virtual directories are created (new-owavirtualdirectory cmdlet). The other roles work fine (mailbox, hub transport).



