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Tumblr to move its half a billion blogs to WordPress
(techcrunch.com)
This is a most excellent place for technology news and articles.
It's a public site that'll be backed up regularly, what kind of important data would you be putting out publicly?
If it's an e-commerce site, than people's payment info, name, and address. If it has a login, then their login information (which they're most likely reusing elsewhere). Even if it's just a static site, than any data that might be hosted on the same server.