Edit: Confirmation!SPOILER ALERT!
Eoin revealed part of the plot at one of his book signings. Apparently Artemis develops a Fairy disorder that causes multiple personalities. Although not all personalities have been revealed we find out from Barnes and Noble Bookclubs that Artemis has become romantic, among various others, and is hitting on Holly(Eoin Colfer @ SciFi Bookclub Forum). When the book comes out we'll find out wether his friends leave him this way... or put him back to the prat he was.