Evan Dodd posted this follow-up post on clustering with E2K3 and AD on the same servers. Bottom line, don't do it. ;-)
"I blogged about KB.898364 briefly when it was created, but over this past weekend there was a flurry of newsgroup posts about putting clustered Exchange onto cluster nodes that are also domain controllers. Several responses referenced KB.281662 (how to cluster AD domain controllers), which didn’t very clearly specify not to do this with Exchange.
That’s been rectified. The More Information section of KB.281662 now clearly states that Exchange is not supported on cluster nodes which are configured as domain controllers:
Note Exchange 2000 and Exchange Server 2003 are not supported in a clustered configuration where the cluster nodes are domain controllers.
For more information, click the following article number to view the article in the Microsoft Knowledge Base:898634(http://support.microsoft.com/kb/898634/) Active Directory domain controllers are not supported as Exchange Server cluster nodes
Hope it helps!"