yet that is. After reading a post made to the Exchange Team Blog this morning I have come to the conclusion that it is highly unadvisable to install Exchange 2007 currently. It was already known that it is impossible to do an inplace upgrade from Exchange 2003 on Windows 2003 to Exchange 2007 on Windows 2003 x64. The main reason give was that you can't do an inplace upgrade from Windows 2003 x86-32 to Windows 2003 x64, however even if you already have Exchange 2003 running on Windows 2003 x64 you still can't do an inplace upgrade. This means you have to build at least a single parallel Exchange server, if not more, to do an upgrade from Exchange 2003 to Exchange 2007.
Now with the above linked post it is confirmed that you can't do an inplace upgrade of Windows 2003 to Windows 2008 on a server that has Exchange 2007 installed. This is due to the fact the Exchange 2007 RTM won't work on Windows 2008. Exchange 2007 SP1 will be required for compatibility with Windows 2008. Exchange 2007 SP1 can be installed on Server 2003, great you say so I install SP1 then upgrade to 2008. No sorry that apparently destroys Exchange in the process due to the removal and reinstall of PowerShell and the migration to IIS 7.0. As a result you will have to again either build a parallel server for the upgrade process or do what is (or at least was) called a forklift upgrade. What this entails is backing up the Exchange databases and other configuration data not stored in AD, then rebuilding the server from scratch with Windows Server 2008, followed by installing Exchange 2007 SP1 and lastly restoring the Exchange databases. All along this process you hope nothing goes wrong.
My recommendation is to wait. Wait until Windows Server 2008 is out the door. Wait until Exchange 2007 SP1 is out the door. Then you only have to worry about parallel servers once rather than twice. It sure would have been nice to know all this at the launch of Exchange 2007 almost a year ago, rather than after I already did the upgrade to Exchange 2007. ARGH!!