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.
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: