Dumbledore gave a kind of reason to Voldemort but it's not the real answer, and that Dumbledore knows something more.
JK said if we are wondering about the overall plot, we should be concentrating on those two questions.
She also tantalisingly answered a question about Aunt Petunia. When asked, JK said no, Petunia is not a squib, as she is muggle-born, but...... and left the sentence hanging.
She said we will find out a bit more about her in the other books and that there's more to Petunia than meets the eye!
She also revealed some more general snippets of information, including:
Aberforth Dumbledore is the barman in the Hog's Head
Dumbledore's patronus is a phoenix
Grawp will be more controllable in the sixth book
We will see a bit more of Rita Skeeter
There will be a new character called McLagan
Harry does NOT have a godmother
The full transcript of the Q&A session will be available on JK's official website soon.