I find that new records are available very quickly, they aren't cached. So using a new dreamhosters.com sub-domain should not be making you wait.
Making a change, if the record is cached the old record does live until the cache expires. That could take up to 4 hours with dreamhost generated records.
Propagation often seems like a verb, an action that is happaning pushing records about the Internet. In fact, propagation is really a noun describing a thing or idea, it's a passive process. It's a little more complicated than this but the basic idea is your computer says "I need an address for example.dreamhosters.com, it first looks in it own cache for a non-expired record, if it doesn't find one it asks it's designated DNS server, that servers checks to see if it already has a non-expired cached record, and if it doesn't then it asks..... And so on. The moral is records are being pulled towards you, rather than pushed towards you. A new record should come quickly, and changed record you have to wait for the cached versions to expire.