It may just be a case of the domain not having propagated fully. Whilst a whois will show you that you have the correct nameservers installed there can still be a delay whilst the domain is pointed to the new IP address. Usually 24 to 48 hours should be allowed to ensure a complete change over.
Check the IP for your domain at DreamHost by going to the Manage Domains page and then click the tiny letters ‘DNS’ and check the IP given at the top of the list next to the letter A (A record).
From your own PC if it is windows XP then run start>run>cmd to enter a dos window and at the prompt type ping example.com (using your own domain) and see if the IP addresses match or not.
If they do not then you can wait a while as mentioned above or if you want to you can use a mirrored dreamhosters.com sub-domain (see here) or if you know how to, place the IP addresses into your windows ‘hosts’ file.