Yeah, in a stereo recording vocals are usually mono, so when inverted and summed the vocals will go off. Instruments and music are usually done in stereo, so it shouldn't be cancelled as much. I might be wrong here though. ;p
The editing can easily be done using Audacity (freeware) or other sound editing softwares.
True, but this is the simplest and cheapest way to do it without using complicated frequency based processing kinda thing. Of course there are many softwares out there can that do better, but I don't know of a freeware that can do that. I guess it depends on how good you want the output to be.