I was reading this on imdb (it's from HPWiki)
[quote]# J. K. Rowling knows exactly what the process for the creation of a Horcrux is, but is not telling yet. The information will be included in the Harry Potter Encyclopedia. She has told her editor what the process is and revealed that the editor felt like vomiting afterwards. All she will say is that a spell is involved, and a horrific act is performed.
# J.K. Rowling described the invention of the Horcrux as comparable to the splitting of the atom: "Something that people imagined might be able to be done, but couldn't quite bring it off, and then... and then people started doing it with sometimes catastrophic effects."
What it could be? She says the editor almost vomit O_O I'd like to know it.