After install Active Directory in Windows Server 2008, you can join your ubuntu client to domain controller.
To join to the domain, you must follow this step (I am using ubuntu 12.04):
1. Go to Ubuntu Software Center, and then you search by "likewise" or "active directory". After that, you can see to package form likewise, but you must choose active directory with GUI. Install and wait for downloading package.
After installation is finished, you must make sure the ip address configuration in the same subnet/network with Windows Server 2008. Make sure with ping in terminal, if replay you can go to the next step.
2. Go to search dialog, type "active directory" and click active directory membership. You must input your password super user to access this application. After that, like image below, you must fill computer name(what you want), domain (fill with your domain name), and the last, click "join domain".
3. If everything is alright, new windows interface will showing like image below. Fill the user and password for server. click OK.
4. Process joining to domain is success!
5. Status domain.