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I've found some odd behavior with the 'anonip' plugin.
It seems to prevent anonymous (i.e. not-logged-in ) users from 'previewing' pages. When they hit the 'preview' button the 'save' option disappears, and a message says that the file is 'read only' (but it's not). If they use the 'save' button directly (i.e. without a preview) they can save the page with no problems.
I've disabled the plugin, and everything's back to normal.
p.s. I'm using Anteater and Monobook.