I'm actually having a decent amount of luck with Audacity... cancelling centred vocals using the 'stereo invert' technique gets rid of the drums too, but that's easily synthed in. And of course there's "Better stand tall when..." which is panned so wouldn't cancel and I dealt with another way (Highlight > Delete!) I've done a rough prototype that actually sounds pretty good! And there's plenty of time to tweak it. But I think I'll drop Northern Cowboy a PM to see what that karaoke version is like, thanks everyone!

Oh and a tip for anyone ever wanting to cancel vocals, you only get decent results from uncompressed files (ie from your CDs!), cos when files are compressed bits that are equal in each channel uncompressed lose their symmetry and so don't cancel properly.