I have just installed Server 2008 and have set up DHCP and it seems to be working fine, until you go to join a computer to the domain. It will allow you to type the credentials in to join the domain but returns with the error "the specified network name is no longer available". Any ideas on how to fix this problem?
I think your problem may be to do with adding both internal and external DNS servers to your scope options. Remove the external entries and leave the clients with just the DC for DNS and see if that helps.