It is practically an article of faith among some Protestants today that Catholics have to refer to them as heretics rather than, as Daniel-Rops, our brothers in Christ. For whatever reasons they do not understand what Catholics mean by anathemas, nor the distinction between material and formal heresy, and consequently when Vatican II speaks of them as separated brethren (Unitatis Redintegratio) they are inclined to view this description as evidence that the Church has “gone liberal” or as mere propaganda.
A few pages later, he writes of another interesting event.
On one occasion Pope Pius IX received in audience an English Catholic priest who, since 1832, had undertaken a campaign of prayer for the conversion of the Anglicans; the Pope was somewhat surprised when, in defiance of protocol, the priest addressed him first, and entreated him to remove the term ‘heretic’ from official texts when referring to Anglicans and other separated brethren. This was done, all Roman documents thenceforth using the term ‘acatholici’ (non-Catholics) (p. 392).
If Daniel-Rops is correct (and I have no reason to suppose that he is not) then the Catholic Church stopped referring to Protestants as heretics around 1851 (when the priest in question, Fr. Ignatius Spencer, made his visit to Rome).
Daniel-Rops shows us that it is no new thing for Catholics to describe Protestants in this way; St. Peter Canisius did it in the sixteenth century, and the Church stopped referring to them as heretics in the nineteenth.
[Update 2012-07-18: Revised a link that went dead]