The AD is already 2003 native, so Schema level 30, fine for W2k3/W2k3SP1...
Microsoft have made a bit of a bodge here however, as the R2 media has TWO copies of the ADPREP utility, etc.
You may already know this, but CD1 isn't just Server2003 SP1, it has been modified slightly to handle the R2 components install but they've left things on there too that should have probably been removed.
CD1 contains the same ADPREP util as you find on normal W2k3 Server media.
CD2 contains another copy, a different one which raises the schema to 31, which is compatible with R2 DC's, and almost completely undocumented. I caught it in the end with another post on here from one of the ever-helpful MVPs...
DC is already installed and running. :) Thanks anyhow!