This is great news!
It parlty comfirms my thoery that the half-blood prince is Salar Slytherin!
The reasons I believe it is him are...
Rowling herself said the themes of Chamber (book two) and half-blood prince (book six) are similar yet totally different storys. So I conclude that the half-blood prince's main theme is going to be about blood! very interesting!
We know blood links (pure blood, half blood, mudblood),blood bonds, life debts (saving someones blood form spilling) is very important issue in the wizarding world!
The main theme is blood by the way!
The Blood bond, wand bond, brother bond, equal bond between Harry and Voldy is the underpinning theme in the series.
In Book five this bond was so important that we could not tell wear Harry began and Voldy end. They now share the same blood thanks Voldy's bulid a body potion in book four.
They share similar backgrounds and many other similaritys and things in common (rule breaking any one!). The list is endless! The only thing that separates them as poeple or wizards is the personal choices they have made!
Rowling also hint that Harry Potter discovers something very important in Chmaber of Secrets which will influnce the whole series.
I think it's that Tom Riddle worships Slytherin's belefies and values.
But we do not find out why Slytherin the founder has these views.
We also do not find out way Riddle who is himslef half blood idenitys with them so passionatly apart from his muggle father.
Why would Riddle a half-blood agree with Slytherin's hard line point of view?
Was Slytherin a half-blood too? I have wonder!
In fact in Book two we learn more about the future Dark Lord and not nearly enough about the man who had a huge influnce on him! Slytherin himself!
Barly any background apart from he set up the Hogwarts with the other founders and had a house named after him. He then had a huge fight with Griffindor about purebloods only been allowed into the school excluding muggle borns (mudbloods) and half-blood ? Defeated he left the school!
No childhood history or what he did after he left. He most have had a wife or Riddle would not be here! We are left with very little about the fight or any other inforamtion about Slytherin. Why has Rowling not told us any more?
Could it be the great Slytherin too was treated like Riddle and Potter by muggles who feared his powers!?! Before he entered the wizarding world as a half-blood? What made him hate Mudblood so? Why did he fear them? We are never told!
This is just my humble thoughs, make of them what you will!