Short answer: No. The Virgin Group constitutes 3 airlines, and although they are all separate, they do coordinate certain things. Virgin Australia has a close relationship with Delta, and if anything, that would push them toward SkyTeam. And United recently cut off their reciprocal frequent flyer relationship with Virgin Atlantic (although i think US Airways till has one). United is pretty strong transatlantic, and a major competitor to Virgin - I don't see them joining forces.