Usually with DreamHost you get one free domain, and then $9.00~$12.00 per year for each additional domain. The unlimited domain thing simply means you can have unlimited domains on your account, but you still have to pay for them.
Some people use a cheaper registrar to keep the their domains registered, and then host the websites on DreamHost. This might make sense if you had loads of domain names, since you could save a good bit of money.
I have about 10 domain names, and just keep them all registered with DreamHost because it is convenient, and is just a few extra dollars a year.