Here's what Dreamhost said:
I sincerely apologize for the downtime you experienced yesterday with your VPS! There was an issue with the RAID controller in the host server for your VPS. We replaced the controller and still found issues, so we will be replacing it again with a new model which should resolve these issues for good!
So, looks like it could happen again. *gulp* So much for arranging a virtual private server to prevent down time.