I do believe the half blood prince may be draco malfoy. it seems appropriate to bring him in to the story further as he continues to taunt halfbloods and become harrys school rival. J.K. Rowling says that at one point she was considering naming book 2 HP HBP but she realized it would be more suitable for the information hinted by the title to be revealed at a later time. also, i havent researched the books to find any info pertaining to malfoys mother, if she is mentioned at all. i guess i just thought the title would be fitting to malfoy to bring hime to eceryone elses level, and away from to "princely" perch he continues to sit on. of course, this is jsut an idea, and i hope to hear others like it.