There have been several clues in different episodes.  The one I keep going back to is the blind lady that had a physical relationship with RJ.  She has said that RJ has a slight build but athletic.  No way could that be Brett, he's got to be too old and wrinkly.  I think the Sheriff McAllister is too old also.  Lisbon and Jane were reading Sophie's notes and said that RJ was an accomplished "whistler".  When Ray Heffner went to visit Lisbon in the hospital after being "stunned" by RJ, he left her room whistling.  Also since RJ has quite the history going back several years, I don't think he would be too young.  If it's one of the people on Jane's REAL list then I vote Ray Haffner.