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Just to see what would happen, we removed my link from those other sites, and sure enough my site dropped to PR 0 after the next update.
Again, just to see what would happen, we re-linked my site from those other ones, and now I'm back up to PR 4 (after waiting again for the next update...).
My advice is to not assume its a penalty, and to investigate how many links you have/dont have pointing to your site from elsewhere.