the placental mammals (eutheria) appear to all share the same gene derived from a retrovirus that is involved in placental formation. This implies that the insertion happened once for all placental mammals. Certain marsupials have very placenta like structures, that appear to be separately evolved; I do not know whether these share the same genetic basis.
I don't know which gene they were discussing but one I am aware of is Syncytin related gene family which is found throughout primates and Glires (rodents and lagomorphs). I have also seen reviews suggesting that there were distinct retroviruses involved in different placental mammals (Volff and Brosius, 2007 (PDF)).
So I was wondering if this was worthy of a discussion of its own and whether Mr. Jack could go into more detail on the genes he was referring to.
On further reading of the Volff and Brosius review I find reference to another placentally expressed gene Peg10 which is thought to have an origin from a retrotransposon of the Ty3/Gypsy class. There are 11 retrotransposon derived genes of this class which are specifically conserved in eutherian mammals and at least 2 of them (Peg10 and Rtl1) are neccessary for placental development in some species (Ono et al, 2006; Sekita et al, 2008).
It is worth noting that there is a distinction to be drawn between a retrotransposon origin for a gene and a retroviral, even an endogenous retroviral, origin.