Don't forget about your friend Google
I believe there are hardware pieces that do this for guitarists, drummers, spooners, accordianists, llama whippers, etc.
search on 'guitar remove vocals
' gives about 14,800 results.
Bah! Not my best search.
Anyway, the 4th result (GETTING RID OF VOCALS (Guitar News Weekly #197, June 10, 2002 - ( ...
) looks promising.
Bah! The site was slow to load so I clicked the 'Cached' link provided by our friend Google
Woo Woo !!! Pay dirt!
Scroll down some and look for the link titled...
AnalogX Vocal Remover (Winamp) v1.03 for Windows 95/98/NT
Click it. (Took awhile to load...)
AnalogX Vocal Remover works on the same principles that the hardware removers do - that in most instances vocals are equally mixed in both channels, and can identified removed by simply changing the phase on one channel by 180 degrees. While this won't remove vocals in all instances, it does work in many cases, and can sometimes be used to remove bass or breakbeat sections as well, which I find to be GREAT for sampling! Depending on the effects used on the vocals, sometimes the reverb or ambience of the vocals is left.
I'll leave further searching as an exercise for the reader.