A provirus is a strand of autosomal DNA that was inserted by a virus once upon a time and got lost in our genome as junk DNA, not being anymore active (would it remain active it'd be a retrovirus). Such insertions are thought to be unique phylogenetic events.
New research has identified a provirus* (HERV-K-Ne1 = HERV-K-De6, inserted in Chromosome 5) shared by Neanderthals and Denisovans but not Homo sapiens. This is consistent with the previous data that placed their autosomal DNA closer to each other than to Homo sapiens.
Lorenzo Agoni et al., Neandertal and Denisovan retroviruses. Current Biology, 2012. Freely accessible (letter with supplementary material) at the time of writing this.
It must be noted however the mitochondrial DNA, inherited by pure matrilineage, is much closer among our species and Neanderthals than either one with Denisovans, what to me suggest that Denisovans are no new species but a hybrid of Neanderthal and Homo erectus. A theory not yet fully testable for lack of DNA from Asian Homo erectus.
Interestingly Denisovans have also several proviruses not found in Neanderthals, what could well support my theory of hybridization. The detected provirus could hence have migrated from Neanderthals to Denisovans in the hybridization episode (along with lots of other autosomal DNA), while the rest could have been retained from the H. erectus ancestors by the maternal line.
However as the article is both very technical and succinct, I can't be sure right now of how strongly or weakly can this info support the hybridization model (founded opinions welcome).
In total the researchers detected three Neanderthal proviruses and 12 Denisovan ones, one of which is shared between both nominal species. It is convenient to remind that while the Denisovan genome was very well preserved and sequenced almost completely, the Neanderthal genome is only known in fragmentary form, amounting to about 60% of the actual genome.