I have had the same problem. I am still somewhat confused though. I am going to move my existing domain but I want to get my site set up entirely first. To accomplish this, I set up a temporary subdomain mysite.dreamhosters.com. I cannot access the database i created over the web for my main domain, but I can for the subdomain.
How then, once I change the DNS servers to point to my new site, do I "rename" the temporary subdomain to my main host? Is this automatic?
P.S. - I did follow instructions to make the subdomain a 'mirror domain' of the main site. So am I correct in assuming that everything I do to the subdomain also applies to the main one?