I heard that OV still requires you to travel with your passport despite the fact that Estonia has entered Schengen. It seems that Estonia and to some extent Latvia hasn't quite understood the "open borders" concept yet.

On a related topic it is always wise to travel with your passport or national ID if your country issues one, even inside Schengen. You are always required to be able to prove your identity to say the police. A swedish drivers license won't do you much good in Spain or Italy if you get in trouble with the authorities...