Domain mapping on Rackspace Cloud Sites

I am trying to setup the domain mapping plugin on Rackspace Cloud Sites, and I have not been able to get it to work. I have read in the forum that you need a dedicated IP for users to be able to setup a domain without me having to do anything, but I do not believe that I have an option for a dedicated IP with Cloud Sites. What do I need to do to setup each new domain for my users?