In terms of schema updates does 'Exchange 2007 SP3'
adprep /forestprep and 'Server 2008'
adprep /forestprep do the same thing?
I'd like to add a 2008r2 DC to an existing 2003-level forest (which requires 2008 adprep on the forest) and I've not yet had the need to apply SP3 to Exchange (which brings 2008 compatibility to Exchange).
Domain functional level will stay at 2003, as all my existing DCs are 2003.