If you're trying to install a Server 2008 RODC and you're getting this error:
A read-only domain controller cannot be installed at this time because default domain groups could not be created. The error was: Unable to contact PDC in domain
... don't forget that your PDC role must be held by a machine running Server 2008. Open up ADUC on a 2008 box and transfer the PDC role to the server and you should have better luck.