It will be: "Harry Potter and the Half-Blood Prince." And in French, "Harry Potter et le prince au sang-mêlé." According to Rowling neither Harry nor Voldemort is the Prince...

More info straight from the horse's mouth at www.jkrowling.com, which is a pretty fun site in its own right!

And another fun fact I learned from this site...Fred and George Weasley share my birthday! Woohoo!