When did Anglican Orders become invalid?  My understanding is that when Henry VIII broke from Holy Mother Church, the newly-created Anglicans were treated like the eastern schismatics, in that their Orders and Sacraments were valid, but illicit.  However, I know by the 20th Century, Anglicans Orders were considered invalid by the Church.  When did this change happen, and for what reason?  Did it have to do with the "ordination" of women, or it predate that?

To start with, please read Apostolicae Curae, Pope Leo XII's encyclical. That is the docuмent which gives the determination of invalidity of the Anglican orders.

Michael Davies says this In "The Order of Melchisedec"

"The 1550 Ordinal was revised in 1552, the principal difference being found in the service for the ordination of a priest in which the ceremony for the delivery of the chalice and paten was removed. Even in the 1550 rite the signification of this ceremony had been reduced to giving authority to minister the Sacraments and not the power to offer sacrifice. "

Father Paul Trinchard, S.T. L. In his book The Abbot and Me on Liturgy (available through Maeta publishers) quotes Abbot Gasquet, whom I believe he says wrote Apostolicae Curae, in his four Advent Sermons given at St. Patrick's Cathedral in NYC in 1913 wherein he explains all the changes that happened to the Anglican Service and also to the Anglican priesthood.

You can find many sources on the Internet for this information. I believe the validity of the Anglican orders were in doubt to a combination of this: due to the loss of the Apostolic line at one point and to the change of the formula for the orders of the priest . Pope Leo XIII made clear that those doubts were confirmed as positively invalid in 1896.

Pope Leo XIII declared Anglican Orders invalid.

True, but it is important to remember that this did not make them invalid.  They were invalid already. Pope Leo XIII simply recognized this fact and officially declared it.  

There are some validly ordained Anglican priests, I expect, but very few.  They are not validly ordained becasue of anything Anglican, but becasue they had been Catholic or Orthodox priests before becoming Anglicans, or were ordained by Old Catholic bishops.  Even then, I am not sure that someone who was really an Anglican theologically could generate valid intention if his life depended on it.  

Many of the arguments against the Anglican Ordinal apply to the Novus Ordo, something that even Michael Davies commented upon in The Rite of Melchizedek though he inexplicably rejects the implications.

As for the Old Catholics, I have been led to believe that they had also changed their ordinal somewhat and their orders may already be patently invalid, though I don't have specific knowledge of the issue; but, I agree that the acceptance of priestesses, by its very nature, means that the understanding of the true priesthood is corrupted and requires one to presume that priests ordained by a bishop who has such a theology is not validly ordained.