What makes a villager spawn?
I have a village in a desert. I took all the villagers out and rode them to my base via a rail system so their village is deserted. The buildings are in one piece each with a wooden door.
I then updated to 1.2 and now no villagers are spawning. I remember reading somewhere that villagers will spawn if they have a "house" to spawn in. I've read the conditions on the wiki and the village houses meet that criteria but no villagers are spawning.
What needs to be done? Do I need to return the villagers and let them "breed"? Do villagers only spawn if they have houses AND other villagers around?
You will need to return at least 2 villagers. Then they will start repopulating the village. As of 1.2, villagers will mate and repopulate based on the number of available houses. See this post (especially the "your village died out" section) for more information.
As of the 1.8 snapshots, though, the villagers need to be "willing" to breed in order to breed. It seems that you can make a villager "willing" by trading with them. Willingness is granted on the first new trade offer, and at a 1 in 5 chance per subsequent trade. Once a villager is willing, they will immediately seek another willing mate, if the village can sustain more villagers (based on the number of houses).
After 3 failed attempts at returning some villagers and then getting slaughtered when I went AFK I managed to get a thriving village of 40+ villagers. My suggestions, fence the area, light it up and sleep in a bed a few times to let the population increase.