Windows 2003, Active Directory and Exchange 2007 setup structure?

Can someone tell me the best architecture/structure to set up a web design/hosting company on Windows 2003 using Active Directory and Exchange 2007?

As a web design company i would be hosting the primary Domain while each client would need their own individual domains configured via DNS.

Does this mean i run dcpromo and add a new domain/forest, or a new domain tree under one forest for each new client. Remember i would need to have complete isolation between each web client, and at the same time would need total control being the primary domain.

A bit lost on the best structure to go with?

Thoughts?