DCPromo and Exchange 2007 - Don't do it!
An interesting tidbit came up on one of the mailing lists I'm on. The gist of it is, do NOT run DCPromo on a machine that is running Exchange Server 2007. It is not supported and it WILL break something. I know this is a small thing, but sooner or later people are going to need to know this. (The same goes for Exchange 2003!).
If you have only one piece of hardware available (that happends to be running Exchange), and you need an additional domain controller, virtualization works well!