Mail sent from Dreamhost domains on shared hosting plans use shared email servers also. The answer as to what is happening is actually embedded in the error message you received:
This is AOL's way of saying that your emails are comming from an IP address that is sending too high a volume of mail to AOL. It sucks. AOL takes the posistion that this is a "valid" AOL response in an effort to control spam, but those arguments completely ignore the fact that just because you are using a shared mail server does not mean you are spamming, and ignore the fact that AOL is preventing their own users from receiving mail they asked to receive and want to receive. Don't be confused on this point: it has nothing to do with the amount of mail you are sending, rather it has everything to do with the amount of mail the combined users of that shared mail server are sending to AOL.
There are numerous lengthy posts and discussions on this subject on this forum (just search for "AOL") which can give you more perspective on this, if you are interested.
I advise that, at least until (and if) Dreamhost/Aol work this out (this comes up periodically, gets addressed, then arises again) you take the same approach as many ecommerce sites, highly interactive news/blogs, and other organizations now do:
Advise your members that, due to AOL's mail handling policies, you can not guarantee that mail sent from your site to AOL members will be delivered by AOL, and advise they use an alternate email address when dealing with your site. Believe me, they are used to seeing that by now.
Irrespective of what AOL tells you about whose "problem" it is (Dreamhost's, your ISP, etc.), that is just a matter of semantics. The "fact" of the matter is that AOL won't deliver your mail because it gets "too much" mail from Dreamhost's shared servers,therefore (apply AOL style logic here) it (all, or much, or most, or some, of it) must be spam.
Like I said, it is a miserable situation, but then again, many things surrounding AOL are miserable situations for their users and for those trying to treat them like full net citizens.