After the outside temperature dropped down to below 20 degrees, my cold air return vents started dripping water out of them. They are located in the ceiling in my kitchen, hallway (2 vents) and bathroom. What is causing this?

It is called condensation. When warm moisture laden air hits a cold surface/cold air the moisture falls out of suspension and if enough moisture falls out of suspension then it will drip.

Below will explain better

Condensation - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Condensation)