I actually don't think Billy would embarrass himself against LeBron, because I don't really think LeBron has learned to outsmart his opponent yet. He just tries to turn the game into athlete vs. athlete, and the veteran vs. rookie factor that Caron Butler used to kill Walker is kind of neutralized in this case. I'm not saying Billy could stop him - even playing that way, LeBron is still the best athlete in the game and nobody is going to shut him down - but I'm not so sure that Bron would destroy him. As athletes go, Bron is in a class of his own, but Bill's not too bad himself.
Let me flag this remark by saying that I don't think Billy should see the floor in the playoffs unless somebody gets hurt (God forbid). I do think Walker would at least survive on defense, but he would be useless on offense - at this stage in his career he can't score on LeBron.