Should an instructor tell his/her students before the exam that there are multiple versions of the exam?
Why would knowing in advance matter to anyone except cheaters?  This information will not help honest students prepare for the exam or change how they approach it, so for them announcing it afterwards seems just as good as in advance.  One argument for announcing it in advance is that some potential cheaters might decide not to try to cheat (and spend more time studying or become a little more honest), but the counterargument is that it might just encourage them to find more effective methods of cheating.  I don't consider either of these arguments especially compelling, and I'm not convinced it really matters much whether the announcement is before or after the exam.  This issue tends to come up in large courses, which are often pretty standardized.  I'd bet the students who care whether there are multiple versions of the exam can easily find out what this professor has done in other large classes, so there usually won't be much of an element of surprise.