Probation can and should be handled on the administrators' noticeboard. Making up a ruritanian committee like this makes probation activities less visible.
For mentorships, I think that insofar as they work well they do so by a very close personal relationship between the mentors and the protege. The mentors have to balance the interests of Wikipedia with the interests of the protege and those of other editors in the community. The mentors in any given instance should be few in number. They have to act with relative autonomy, and committees and whatnot would hamper this.
Earlier activities such as those mentioned further up this thread, and the formation of a password-only channel for discussion, have been reversed. Good.
I think Kelly's comment about Wikipedia becoming a high school rings true, but we don't need to encourage this process.