David Heddle has posted some thoughts on the Protestant principle Sola Scriptura.
I've often wondered how Protestants reconcile this principle with the canon of the New Testament. I.e., how can one accept the inspired nature of Sacred Scripture without recourse to an extra-biblical authority (namely, the magisterium). Hopefully David will deal with this argument in the near future.
In the meantime, read Gary Hoge's excellent presentation of this argument against sola Scriptura, as well as the follow-up. I'd also recommend Gary's response to James White's attempted rebuttal of this argument.
Update: David pointed me to a prior post of his, in which he dealt with the canon. I was somewhat surprised to note that David posits that the canon is a fallible list of inspired texts. If that were true, it seems to me that ultimately, there is no way to prevent someone from removing texts from the NT, if they provided sufficient "evidence" for their actions.