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Adding new pages/content to your site won't cause a drop in page rank, unless, the new content is what causes other sites to remove thier links to your site.
And that happens.
If you had any rented links that expired that could certainly cause a drop in PR, if those links passed PR.
Does PR vary across datacenters? Indexes? My understanding is that it is updated periodically (around the same time link update takes place) and not in constant flux. My new pages have gone from PR0 to a PR4 about 3 months later.
In your case I think it is unrelated to the new pages.
A little disheartening but it was probably on the cusp of 4 - 5 PR anyway. Might also have been due to converting the site to XML a few weeks ago and assuming google would index it properly (what a mistake that was). It was changed back to HTML soon after we realised.