Unique IP addresses are attached to a particular domain, so you should be OK in the situation you outlined.
Something else I have noticed (not quite on topic); I have a few domains hosted on DreamHost, all on the one server, and I have not purchased unique IP addresses for these domains. However, each of these domains resolves to a different IP address.
I am not sure how many IP addresses each DreamHost box has (besides the uniques purchased by customers), but it seems the odds of most of your domains being on different IPs is quite good. This could be handy for SEO purposes.
You shouldn't have too many problems with that.
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