Q:

Is it correct to regard archeaic humans (i.e.n Neanderthals and Denisovans) as distinct species to Homo sapiens?

There is increasing evidence (e.g. here) that Homo sapiens interbred with contemporary homonins such as Homo neanderthalensis and the "Denisovans". Although these homonins do show differences in their mtDNA, since a species is generally defined as a group of organisms capable of interbreeding and producing viable offspring, should we not regard humans, neanderthals and denisovans as the same species and name them as such?

A:

This question highlights the different species concepts. 
If you go by the reproductive species concept (a population of actually or potentially interbreeding organisms), then yes. Apparently, the populations were interbreeding.
If you go by the typological species concept (morphologically distinct), then no. Homo neandertalensis and the "Denosovian" are morphologically distinct enough to be recognized as such.

