As mentioned in another thread, it is a good idea to add the domain to the hosting system before changing the name servers to ns1.dreamhost.com etc. This prevents any possible domain hijacking by your fellow DreamHost customers, not that this is likely to happen, but it is better to be safe.
Also, if your domain is of the type .com.au or .co.uk etc. you may (or may not ) need to take an extra step or two when adding the domain to the hosting system. If you have any problems with this, post back here and I'm sure someone will detail the steps required.
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