If you sign up for a 2 week free trial then you can either:
-- register a domain name, and pay up front for the domain registration, if you continue after the 2 weeks you can use that domain as your free domain, and the registration you already paid will be deducted.
-- register a domain name, and pay up front for the domain registration, if you end your hosting service after the 2 week free trial, you still have use of the domain name for 1 year. (After cancelling your hosting you will still have access to the dreamhost panel but with only options for managing DNS and registrations.) You can point your domain name to any host.
-- not register a domain name and use one that you already own, or use a free *.dreamhosters.com sub-domain.