Brad Rutkowski: Currently we are running Win2k3 SP1, R2, SP2, Win2k8 Beta3, RC0, RC1, and RTM Escrow idomain controllers in production...  Since we're running some downlevel servers in the environment and I was interested in what is supported to be upgraded to Windows 2008 when we sign off and the the DVDs start getting pressed.  Luckily a mail came by on one of our discussion aliases that has the details I (we) desire.

• You can have DCs with down-level OS down to Windows 2000 SP4 in the same forest along with WS2008 DCs.
• This means you can have forests with a mix of WS2008, WS2003 SP2, WS2003 R2, WS2003 SP1 and Win2K SP4 (please have in mind that this depends on the forest and domain functional levels).
• If you have a down-level only forest (i.e. no WS2008) and want to introduce a new WS2008 you will need to run ADPrep (ForestPrep and DomainPrep).
• You can run ADPrep having down-level OS down to Win2K SP4, you don’t need to have all of them with WS03 SP2.
• However if you are going to in-place upgrade any of the down-level DCs, these have to be at least WS2003 SP1.