I googled "iTunes copyright infringement" or something like that, and came up with this link:
So I filled out the forms and all, and sent the following explanation along with it:
These individuals have used MY cover performance of the song She Sells Sanctuary by the Cult and represented it as their own. I did not grant them permission to do so. My performance, which predates theirs by two years, is available here without the vocals they later added: http://www.youtube.com/watch?v=O8ZjJzB33ZQ Clearly, mine is dated in September, 2011, and clearly it is the exact same performance that they have only added vocals to. Thank you for your prompt action on this matter.
I can't remember when I exactly sent that, but Apple Internet Services Legal replied to me quite quickly, a matter of a day or two, on April 12. They said they'd contact the provider (Routenote distribution service) and give them 5 business days to respond. If Routenote confirmed my claim, they continued, they'd remove the song, but if Routenote stood up for their thieves, they'd leave it up to me and Routenote to figure things out.
Lo and behold, five business days later, I received a message from iTunes legal telling me that they had removed all of the content from the iTunes store. I checked - Not only did they remove the song that I asked them to, but they also removed the whole album it was on! Nifty! This is progress. But clearly I still have to go after Routenote, who appear to have distributed the song in the first place, if I wish to eradicate it completely. But I'm feeling lazy. I'll rest on my laurels and give it a rest for a few days.