Correct. You can add unregistered domains to the DreamHost system as 'fully hosted'. However, they will not be reachable until the domain has been properly registered and the DNS information has propagated across the Internet.
If you need to access the domain before registration and/or DNS propagation has completed (eg: for testing), you can use a technique detailed in the following wiki article.
Basically, you create a free dreamhosters.com sub-domain and set it to mirror your real domain.
Save [color=#CC0000][b]$50[/b][/color] on DreamHost hosting using promo code [color=#CC0000][b]SAVEMONEY[/b][/color] ( Click for promo code details )