I am guessing that the net effect of all these actions by Madonna, Dixie Chicks, and other entertainers is probably fairly positive for Bush. Major segments of the music and film industries are making themselves took worse and worse and making Bush look like more and more of a hero for sticking to his principles without engaging his vicious detractors directly. This strategery is working pretty well, IMHO.
I think not backing down, yet not replying publicly to abuse is a very deliberate plan on Bush's (and perhaps Rove's?) part. It may well be the best public response possible to looney leftists.