I have a few mirror sites and I was just wondering if the search engines view these as separate sites or one. Also, does it hurt your PR having these mirror sites?
I think you should consider instead setting up those mirror sites on their own host, and develop them as doorway pages linking to your main site. This will help boost your rankings.
The different websites you have, do they all have the same content? If yes, then that is duplicate problem and will cause some serious issues. You should not have the same content on multiple sites. And incase you have unique content on all sites and you are worried about having all sites on the same host, then I just you do not worry about it anymore. You can have multiple sites on the same host/IP and there is no problem of any kind with that.
They won't hurt or help. Google will consider it duplicate content the same as if your article is reprinted.