Check out the AD Migration Manager tool from Quest.
There is also the AD Migration Tool from Microsoft that might assist you. See the Microsoft ADMT guide for additional details
For a read-only domain controller, you don’t need to worry about users other than caching login information. DNS and DHCP are setup just as they would be in Server 2003; install and configure the services. The file shares are the same, copy the files over and create shared folders.
One thing you need to know, is you need to already have a full windows 2008 domain controller on your domain.