Reuters is reporting that the Russians have stationed tanks, artillery and military personnel in Syria.
It is building up the airfield at Latakia and one near the port of Tartus where its ships are based.
If, as seems likely, Russia sends aircraft to attack IS we would have Russia, US, NATO and a coalition of Arabs all operating in the same battle zone against a common enemy, IS, but supporting different sides in the civil war. They will all have to communicate and cooperate with each other to avoid 'friendly fire' incidents, and that could prove a good thing.
It could also go the other way if things go wrong. Interesting events.