They're mentioned everywhere in Creatures Anathema, can be sanctioned or unsanctioned, but nowhere is mentioned what they actually are.

Vat grown psykers? Perhaps using cloning technologies?

This question has been asked twice before. Maybe you can find an answer in either of those threads (haven't read them myself so I can't answer):

http://www.fantasyflightgames.com/edge_foros_discusion.asp?efid=70&efcid=3&efidt=98602

http://www.fantasyflightgames.com/edge_foros_discusion.asp?efid=70&efcid=3&efidt=87515

Maybe we should pin the main vat psyker thread?