Hooray for the Exchange Pre-Deployment Analyzer. If you've not seen this, then check out the tool here. This is a tool for checking that an existing environment is ready for Exchange 2010. I ran this tool recently and it discovered a couple of Active Directory issues that I probably wouldn't have spotted.
Specifically, the tool reported a permissions inheritance block on 3 separate OUs, one of which was the Domain Controllers OU.
Definitely a tool to run before you hit Exchange 2010 setup.