Here’s an update on what to expect of Office Communications Server 2007 & Windows Server 2008
OCS 2007 will not be supported on Windows Server 2008.
Windows Server 2008 domain will work. You will need to re-run to AD prep tool after upgrading you Windows Server 2003 domain to Windows Server 2008 AD Services.
In new domain environments (no upgrade scenario) there should be no problems installing OCS 2007, prior to completing the AD prep.