Actually, there is a way to install an easy one-click in the actual domain rather than a sub-domain, but it is not intuitive at all. If you already have the domain
"hosted" at DreamHost (you see it in the Domains -> Manage Domains" screen), then delete the hosting by hitting the "Delete" button under the "Web Hosting" column for the domain.
Then you will be able to specify the domain for the "one-click" install to use, and the result will be that your Drupal will be at the "base" of the domain.
--DreamHost Tech Support