It might be a little too early to tell, but are there plans to leverage the Snow Leopard Server features? Broadly speaking, there's a lot of overlap in functionality, but I also see a lot of opportunity to leverage the native services.
I have no experience running ZCS from a Mac OS server, so I'm not even sure to what degree the 10.5 services are used (does ZCS simply replace them with the "standard" set?).
Licensing wise, I gather that 10.6 server will incorporate Outlook compatibility and some form of push mail (Activesync?), so those parts could possibly be omitted in that particular NE build.