I am migrating our users from SBS 2003 to Server 2008 and Exchange 2010. I have built and joined a 2003 Server to the domain, have the exchange 2010 server coexisting with the Exchange 2003 server, ran adprep /forestprep and adprep /domainprep on the SBS 2003 domain, but when I run dcpromo on the 2003 server its stills saying I need to prep the domain-forest. I went back and tried to run it again on the SBS2003 domain and it says the domain has already been prep'd. The functional domain level is as high as it can be at Windows 2003 so I am not sure what else I am missing here. Any ideas?

Nevermind, I figured it out. I was running the adprep /forestprep from the SBS2003 Server's I386 directory (Duh) - you have to run it from the Server 2003 R2 disk. All is well!