I'm not sure that's the case -- something's been slow lately.
I've been adding hosts to a new domain, and normally I see new DNS records propogate, at least within Dreamhost, within a few minutes.
Lately, it takes forever for Dreamhosts's DNS to update. Even new records are taking up to a couple days to show up.
For example, I set up a mailing list yesterday, and "lists.mydomain.com" didn't exist until about 24 hours later. Today I added a MySQL database, and several hours later the new, unique hostname isn't visible, even from inside Dreamhost.