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Update: The bug has been found and fixed.
A disturbing security hole has been discovered by a user of the mobile site for Facebook.
No matter what level of privacy you have set on your email address, it is visible on the contacts page (seen on a Windows Mobile device). No word as of yet if it this error can be replicated on phones running other operating systems, but it wouldn't be a surprise if that were the case.
This is a recent discovery, and shouldn't be too difficult to rectify once the Facebook security team gets on it. But until the problem is remedied, it's free-for-all on stalking your exes.