Does it still exist under manage domains?
Sometimes the DNS records are deleted but the physical folder stays in your account. Have a look with an FTP client and see if it has gone. If it is there delete the folder i.e. delete subdomain.example.com. However if it still contains a log folder you will not be able to delete the subdomain folder because the log folder belongs to root.
If you still are unable to add your domain then send in a support request. It may just have been caused by some recent fileserver problems and is still in the queue.
Opinions are my own views, not DreamHosts'.
I am NOT a DreamHost employee OK!! :@
You act on my advice at your own risk!