As a precautionary message, DreamHost has reset their customers' passwords.
Sadly, no information has been forthcoming as to how a hacker might have been able to gain access to one of DreamHost's internal servers. Hopefully the firm is investigating and fixing any potential security vulnerabilities which might expose customer information in future.
Of course, if you use the same password elsewhere on the net (a very bad habit from the security point of view) you should make sure that you change it now. After all, if hackers do have your DreamHost FTP password you don't want them to be able to log into your email, eBay, Amazon, etc accounts too.
Remember, your passwords should not just be hard for people to guess and difficult to crack, they should also be unique. That way, if a password is lost by one company (as it may have done in the DreamHost case) at least there's no knock-on effect to your other online accounts.