One option would be to use the 'Domain Mapping' provided by wordpress.com, as explained in the article linked below;
Basically, you need to change the name servers for your domain to...
To change the name servers for the domain registered at DreamHost, go to Domains -> Registrations in the DreamHost panel, tick the checkbox for the domain in the Modify Whois? column, click the Modify whois for selected... button and you should see the section for changing the name servers at the bottom of the page. After changing the name servers it will take up to 72 hours for the changes to propagate across the Internet and your site to be reachable under your domain.
Please note: The wordpress.com 'Domain Mapping' service appears to be a paid service, so you might be better advised to add a hosting plan to your DreamHost account and host the blog here. Transferring the contents of your existing blog shouldn't be too hard.
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