Deployed our firt CAS/HT 2010 in production in Exchange 2007 envoirenment. All went well. Ran the ExBPA on CAS2010 and it tells me PDC emulator is not excluded from DSAccess topology.
I would just ignore that warning. The warning does not consider how large your environment is and what the actual load against the domain controller is. For 90% of the organizations there's no need to exclude the DC with the PDC Emulator role.