[please see the reconsideration of this post here]
This morning coming in to work, I heard Andy Barrie on CBC say something like "How is the water post-Walkerton?" I've forgotten the actual sentence, but it ended "post-Walkerton." (See here for an explanation of the Walkerton Tragedy.)
It struck me that post-Walkerton sounded, prosodically, like one word rather than two, and that, if it were one word, it would be a preposition (albeit an intransitive one, explained briefly here.) The Cambridge Grammar of the English Language claims that pre- and post- can form adjectives, nouns, and verbs, but doesn't allow for the possibility of prepositions. Clearly though, the word post-Walkerton as used above is neither adjective, verb, nor noun.
The other way to look at this, I suppose, is that post is itself a preposition rather than a prefix.