Q&A at Rowlings' Site
J.K. Rowling decided to answer a few questions at her official site, concerning the upcoming book.
Here's what she had to say:

Will Arthur Weasley be the new Minister of Magic?
Alas, no.
 
Is Tom Riddle the Half-Blood Prince?
Well, as Tom Riddle is the same person as Voldemort, and Voldemort is NOT the Half-Blood Prince...do I really need to answer this?
 
Why are some people in the wizarding world (e.g., Harry) called 'half-blood' even though both their parents were magical?
The expressions 'pure-blood', and 'half-blood' and 'Muggle-born' have been coined by people to whom these distinctions matter, and express their originators' prejudices.  As far as somebody like Lucius Malfoy is concerned, for instance, a Muggle-born is as 'bad' as a Muggle.  Therefore Harry would be considered only 'half' wizard, because of his mother's grandparents.
If you think this is far-fetched, look at some of the real charts the Nazis used to show what constituted 'Aryan' or 'Jerish' blood.  I saw one in the Holocaust Museum in Washington when I had already devised the 'pure-blood', 'half-blood' and 'Muggle-born' definitions, and was chilled to see that the Nazis used precisely the same warped logic as the Death Eaters.  A single Jewish grandparent 'polluted' the blood, according to their propaganda.
 
In what way is 'Harry Potter and the Half-Blood Prince' related to 'Harry Potter and the Chamber of Secrets'?
I have been engulfed by an avalanche of questions on the subject of 'Prince' having once been a title of 'Chamber'.  I am therefore attempting to answer most of them under this heading, which I think just about covers all the answerable variations (the unanswerable ones include questions such as 'who's the Half-Blood Prince?' 'what happens in the  Half-Blood Prince?' and 'what does Half-Blood Prince mean?')
 
The plot of 'Prince' bears no resemblance whatsoever to the plot of 'Chamber', nor is it an off-cut of 'Chamber'.  The story of 'Prince' takes off where 'Phoenix' ended and does not hark back to four years previously.  True, mention is made to events that happened in 'Chamber,' but of course mention is also made of events that happened in 'Stone', 'Azkaban', 'Goblet', and Phoenix'. 
 
'The Half-Blood Prince' might be described as a strand of the overall plot.  That strand could be used in a whole variety of ways and back in 1997 I considered weaving it into the story of 'Chamber'.  It really didn't fit there, though; it was not part of the story of the basilisk and Riddle's diary, and before long I accepted that it would be better to do it justice in book six.  I clung to the title for a while, even though all trace of the 'Prince' storyline had disappeared, because I liked it so much (yes, I really like this title!).  I re-christened book two 'Chamber of Secrets' when I started the second draft.
 
The link I mentioned between books two and six does not, in fact, relate to the 'Half-Blood Prince' (because there is no trace left of the HBP storyline in 'Chamber'.)  Rather, it relates to a discovery Harry made in the 'Chamber' that foreshadows something that he finds out in 'Prince'.
 

Well!  That's it!  That's what bit of info and clues she has given us for now on her upcoming book.