After spending some time thinking about the question regarding whether you should deploy Exchange 2007 SP1 on Windows Server 2003 or Windows Server 2008, I decided to do some more research on it. 

I came up with the table below and I am including the public sources of my information as well.  In short, using Windows 2008 could provide a faster migration experience as well as performance improvements.  I am actually preferring Windows 2008 now.  Hopefully this will help you in your decision. 

Windows 2008 Windows 2003
+ Faster installation time – No additional downloads or prerequisites required.  Additional components can be installed using an XML file. + Familiarity of Windows 2003 – less learning time
+ Already Running Windows 2008 and no need to upgrade at a later date.  + Already a stable base OS for many customers
+ Performance and scalability enhancements for Client Access server - This requires installation of a number of additional hotfixes and Service Packs to get Exchange 2007 SP1 installed
+ Multiple subnet failover clusters – support for SCC or CCR in multiple Datacenters without the need for a VLAN to span subnets - CCR requirement for same subnet restricts options.  SCR can be done across different subnets however.
+ More Quorum configuration options for clustering. - There is no supported upgrade path from a Windows Server 2003 running Exchange 2007 to Windows Server 2008
- Existing Exchange 2003 servers can’t use Windows 2008 domain controllers that are read only.  
- Exchange 2003 may not install correctly in a pure Windows Server 2008 forest if you try to install Exchange in a child domain without installing Exchange in the parent domain.
- The big unknowns – The “Wait until SP1 rule” that many have.  

This is not by any means a complete list of all of the benefits of going to Windows 2008.  This is only what I have found publicly so far as Exchange 2007 SP1 goes.  As I spend more and more time with Windows 2008 I am really liking it.  I like the new interface for managing roles and features a lot. 

Sources of material: