It's scary, Apple have some ninja **** on their DRM. I have a program called Noteburner M4V Converter
to be able to do it. And here's how it fucking did it; it would play the movie and screen capture the entire thing and make a new file out of it. Sounds like the most roundabout way possible but Noteburner got away with charging $40 shareware for the privilege.
But that was over a year ago, Apple's DRM has evolved since then.
Maybe you could record, with a camera and tripod, iTunes playing the file.
UPDATE!: I downloaded their latest version and it WORKS again!