Just my 2 cents, though it may be a bit redundant to what's already been said.
My understanding is that the Pope speaks infallibly when he specifically invokes the power of Peter to "loose and bind" or "strengthen the brethren", as John Paul II specifically does in _Evangelium Vitae_ (declaring that life begins at conception) and in the 1994 encyclical on ordination (I always forget the title, infallibly declaring that the Church cannot ordain women).
But people seem to limit "papal infallibility" to declarations. Like, "The only times popes have speak infallibility have been the declarations of the Immaculate Conception and the Assumption as dogmas."
Of course, those were the two theological dogmas declared *after* Vatican I declared infallibly that the Pope can speak infallibly on his own (a declaration which Vatican II later finished defining).
An "infallible" declaration is only necessary when a) the Church has never definitively spoken (e.g., Immaculate Conception or b) the Church has never addressed the question(e.g., when life begins).
Now, I've heard that women's ordination was addressed by one of the early Councils, anyway. But if John Paul II's encyclical was not "an exercise of infallibility," the only reason would be that the issue had already been infallibly defined, and he was just reiterating that definition. (He certainly uses the formula).
But we must not forget that a Pope speaks infallibly when he reiterates something that the Church has already infallibly declared.
Now, _Humanae Vitae_ *does* change Church teaching. It allows NFP to be publicly promoted (Casti Connubbii only allowed it to be discussed privately between priests and couples--advice I personally thing the Church should have kept to). It specifically addresses the issue of chemical contraception.
Now, contrary to popular belief, chemical contraception was not an invention with the Pill. So it's hard to say whether the Pill really presented a *new* situation.
If the Pill represented a new situation taht the Church had not previously encountered (which many people believed and continue to believe), then--even if the previous teaching on contraceptio nwas infallibly defined--a new situation existed, and a new definition would have been necessary.
However, if the pill did *not* represent a new situation, then the previous teaching applied. *Then* the question is whether the previous teaching could be considered "infallible," and I think it can.
