Providing the vocals have been mixed dead center of the stereo image then it is possible do scrub them out in a programe like Cubase. As others have already suggested, it's by using phase cancellation.
Import the track you want to use as a stereo file into Cubase (or your sequencer of chocie) and invert the phase of either channel. All instruments that have been panned off center should remain intact, but vocals and anything dead center will be scrubbed. It's not the most natural sounding way of doing it as you end up with the slightly hollowed out "kareokee" backing track effect, bit it's the only way I can think of doing it. I used the technique on a life recording of "Run to the Hills" a few years back when a mate wanted to record a pastiche vocal line. Worked pretty well if I recall...
Hope that helps mate
*Rahnooo*