Here's a recent map showing ethnic distribution:
http://rreu.11net.com/Genealogy/images/maps/image9.htm
Not sure when this one is from:
http://www.anesi.com/rmap2.jpg
This one is similiar to the last one
http://www.cjcr.cam.ac.uk/resources/maps/Ethnic16.gif
I had another really good map earlier, but I can't find it now.
There's still a very strong Hungarian influence in Transylvania, even despite these 85 years under Rumanian control. There's a strong Hungarian minority in western Transylvania, and there has always been a solid majority in the eastern sections of it. Transylvania could probably go either way. However, the difference is that Hungary lost two wars, while Rumania only lost one.

Peace treaties and border shifts can cause large population movements (Prussia, Pommerania and Silesia anyone?).
Also, Macedonia was given to Bulgaria by the Germans during the war, along with the Greek coast from Thessaloniki eastwards to Thrace as well.