half blood prince

[Fiona]

No: HPFGUIDX 107685

What do we know for sure about the HBP?

JKR has confirmed its neither Harry nor Voldemort.
JKR said that it had been the working title for book two.

My theory is that the HBP is Gilderoy Lockhart.

JKR brought him back in book five, and it is possible to argue that 
Gilderoy relates to a "Gilded King" and he's certainly foppish enough 
to be like some Prince, well divorced from reality.

Just a theory....  any opinions?