Console session: https://gist.github.com/3355357
Take a look at the query for the update (at line 44). Seems MongoDB is rewriting my query, but it leaves the original query in place. (See also the comments on this gist; there's some insight in the code, but not much.)
The reason I stumbled upon this is that I was trying to do some scripted explains on my profile data, and got something like this: http://paste.roguecoders.com/p/efd3829bba5ab5fd19ba8bb1a073da6a.txt
Note the second clause (which should be entirely redundant because Mongo is selecting one explicit unique record (the _id)). This gives me an empty BasicCursor.
From here: https://groups.google.com/d/topic/mongodb-user/we4kDRdE4GM/discussion (Kristina and Derick both suggested I file this in Jira).