[The Saucepan Man]

Saruman's army did feature Wolfriders. In UT, in The Battles of the Fords of Isen, it is said of the army that attacked Theodred's forces:

In its van were some Dunlending horsemen and a great pack of the dreadful Orcish wolfriders, feared by horses.

and

But before them, swift and silent, went several troops of the dreaded wolfriders.

The wolves here are not referred to as Wargs. But given that we are told in LotR (A Journey in the Dark) that the wolves which attack the Fellowship in Eriador are Wargs which have "come west of the Mountains", it is not unreasonable to assume that Saruman's wolfriders were in fact Warg-riders. (In that sense, the Warg-rider scene in the film did have a grounding in the books, even if the "Wargs" looked more like hyenas than wolves [img]smilies/mad.gif[/img] ).

I doubt, however, that the wolfriders referred to in UT were Uruks, as the Uruk-Hai in Saruman's army are distinguished from run-of-the-mill Orcs and the wolfriders are simply described as being Orcish.