When a DC has no connectivity to the domain, and you want to remove it, you have to boot it up in DSRM and run dcpromo /forceremoval
Now according to technet (
Forcing the Removal of a Domain Controller ) , you can easily clean up the forest metadata by just removing the DC object in ADUC in 2008 and R2.
If you use the version of the Active Directory Users and Computers snap-in that is included with Windows Server 2008 or Windows Server 2008 R2. or the Microsoft Remote Server Administration Tools for Windows Vista, you can clean up the metadata automatically by deleting the domain controller object.
While in 2003, it used to be a much longer process
How to remove data in Active Directory after an unsuccessful domain controller demotion
What I want to know is if the first technet link is correct, and it is really that easy in 2008/R2, or is there an error in that link?