I agree with what you are saying as the ideal (and eventual) behavior of the system.  But I don't know what the current state of the art is in bitcoin client and miner code.  Are these systems currently vulnerable to such shenanigans?  Or are protections already in place?

I have to say that my reading of the 'official' bitcoin client code revealed a very, very disturbing lack of defensive coding.  Not encrypting the wallet file was an obvious and glaring omission but there are innumerable other aspects to the code that make it look much more like a prototype and much less like a client written to handle money in a trustworthy fashion.