Under normal circumstances, you get one CAL per user or per device on your network; you really don't want to deal with the complexity of per-host CAL licensing. Really.
If you have Windows 2008 CALs for all your users or devices then you don't need anything extra for your VM host, but you probably know that already.
A user/device accessing a non-2008 VM that is hosted on a 2008 server does not need a 2008 CAL. If the guest OS is a Windows Server OS, then they need a CAL for that version of Windows.
If your situation is like ours - lots of 2003 CALs, no 2008 CALs, no SA, then yes, you can run 2003 servers virtually on top of 2008 Hyper-V; just get enough 2008 CALs for your admins.