Update at Rowling's site
J.K. Rowling has updated her own site, as to comments and replies 'made by her' on fan sites. She wrote the following:

Well, here we go again! The gossip, the non-stories, the ever-wilder speculation...
Please remember that allegations of 'leaks' concerning deaths in 'Half-Blood Prince' are very easy to make. Any comment I make on such rumours would be a spoiler, so I'll just repeat: keep those barrels of salt handy. Speaking of which...

I have been alerted to the fact that I am supposed to have posted in the 'comments' section of a couple of different fan sites recently. I never post on fan sites and it is important that you all know this, because the last thing I would want is for any of you to be misled or otherwise manipulated by somebody posing as me online. JKRowling.com is the only place I ever post. Lest you think I'm protesting too much, please ask yourself whether I would really want to reveal my email address on a fan site. Love you all though I do, I don't really want to wake up tomorrow and have to download 21,956,038 new emails, all demanding the identity of the Half-Blood Prince.

So there you go everyone. As we said, there are bound to be many of these rumor reports, such as the betting story from a few days ago...so keep that salt handy! Only 50 more days to go!

And as for one recently dispelled rumor about her sixth book, she's written the below in her 'Rubbish Bin'.
JKR has said that 'Half-Blood Prince' is her best book yet
No, she hasn't. She is superstitious about saying things like that! What she said - on this very website - was that she had not been this happy with a book since she finished 'Prisoner of Azkaban'.