I usually refrain from commenting when it comes to domain registration: since these folks at DreamHost (Jeff, Will, etc.) are soooo nice, so even I register ALL my domains elsewhere and never even take advantage of their free domain offer which is included, I rarely respond to threads related to the industry that I know best.
However, both TorbenGB and dtobias might have left out one thing: that DreamHost is not just another web hosting company which also registers domain names for you, they are an ICANN-accredited registrar themselves. And with that your domains are under better protection as there are guidelines to follow in the contract between ICANN and their registrars. So even when DreamHost goes down (which I too honestly don't think would happen in the foreseeable future), your domains should be fine.