HBP still leaves many questioning
A story done in the Newsday (US) has a piece done, showing the most frequently still unanswered questions, as well as theories to many of them. It was taken from HP boards all over the web.
One small part of the article can be found below:
Do those initials belong to the now-dead Regulus Alphard Black, brother to Harry's godfather, the much-mourned Sirius? It's a conclusion Rowling herself has called "a fine guess." Since breaching the cave's magical spells requires two people, only one of which can be a wizard, many have speculated that the Black family house elf, Kreacher, helped Regulus retrieve the original horcrux-locket.

In "The Order of the Phoenix," a heavy locket that cannot be opened is found among the Black family possessions. Hmmm.
The full article can be found HERE.