Look your domain up with a 3rd party whois server such as: http://www.networksolutions.com/whois/index.jsp
and check what nameservers are listed there.
Also you can tell if propagation complete using the tool: http://www.whatsmydns.net
That one's tricker to read tho, because you also have to check who's ip (old host or dreamhost) that it's showing green tick marks for.
If you give us the domain name we can look it all up and tell you if anything is wrong.
DNS and nameservers changes don't happen instantly. they normally are done sooner that 12 hours, but can take 24.. and it's possible but not likely to take longer worldwide. Not everyone gets the change at once either.