"Accusing a Muslim or a Muslim group of disbelief is in itself an act of disbelief. The prophet never accused even the individuals he knew to be hypocrites of disbelief, nor did he treat them in a way any different from other Muslims. No good can come from Muslims accusing Muslims of disbelief; this can only lead to arguement, resentment, and further division. If you find a Muslim saying something which you believe to be in error, present to him/her your reasons for believing them to be in error and try to help them see the truth. Don't waste your time squabbling over petty differences."
A posting by the "American Taliban," John Walker to soc.religion.islam in June, 1997. (He's 20 now, so he must have been 16 when he posted this. By the way, I found this via Robot Wisdom)