For what it's worth...
This past Wednesday, I called Apple. I told them that I had bought Applecare for my MBP on the internet, and not from Apple. I asked them to confirm that my number was legit, they looked it up and said, "yes, you're covered, no problem, it's legit". I then said I didn't receive a box from the seller only a code that was emailed to me, and asked if that was ok. They said "that's fine, that's not a problem at all, you're all set."
Now, I feel, that if there was an Ebay Applecare issue, and with the tons and tons of Applecare being sold on Ebay, you'd think Apple would be aware of this (if it were not legit in some way), and their tech support folks would be aware of such a thing. So if a customer called them to ask about his Applecare number/reg, and if it was legit, they'd have some sort of response about what to look out for. I mean, the person I spoke with kind of sounded like I was just being a worry-wort about it, and that I really was "all set". Just my experience, but my thinking is, there's a lot of fear mongering going on about this, and I don't understand why.
I suppose time will be the only true teller of whether there is any truth to this phantom Applecare Keygen out there. I did do a search on all/most/(all that I know of) crack/p2p/filesharing sites, and could not find a single hit for Applecare, or Applecare Keygen.