In this case, it probably just means your site’s been moved to a different server since the last time you SSHed into it. SSH kind of freaks out when this happens, since it can’t tell the difference between that and someone trying to trick you into connecting to the wrong server.
You can either remove the appropriate line from .ssh/known_hosts, or just remove the whole file, if that’s easier*.
*: [size=x-small]To the nitpickers: Yes, this will remove known_hosts entries for other servers as well. This is a slightly unsafe shortcut, but it will be fine in most cases, especially if you’re only using SSH to connect to DreamHost anyway.[/size]