With the release of Exchange 2007 SP2 we are providing a supportability matrix that identifies the various supported configurations of Exchange Server (operating system, Active Directory, Clients, .Net Framework, etc). You can find this document on our Exchange TechCenter at http://technet.microsoft.com/en-us/library/ee338574.aspx. Here are some notable guidelines from the article:
Exchange Server 2003 is not supported in Active Directory environments whose domain/forest functional levels have been raised beyond Windows Server 2003. However, Exchange Server 2003 SP2 is supported against Windows Server 2008 Active Directory servers.
Exchange Server 2007 is not supported on Windows Server 2008 R2. However, Exchange 2007 SP1 RU9 and SP2 are supported against Windows Server 2008 R2 Active Directory servers. In addition, Exchange 2007 SP1 RU9 and SP2 are supported in Active Directory environments whose domain/forest functional levels have been raised to Windows Server 2008 R2.
Exchange 2007 SP2 is supported with PowerShell 2.0.