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).

Yes, you need to return them. Villagers do not randomly spawn, but only come into existence in one of these ways:

• When the part of the world containing the village is generated.

• By two existing villagers breeding.

• As a “cured” Zombie Villager. This is a multi-step process but is the only way to get a villager from nothing (Zombie Villagers spawn randomly like other monsters).

• Commands, spawn eggs, etc.

Villagers will mate depending on available housing. The amount of housing can be supplemented by the player placing wooden doors nearby. To allow Minecraft to recognize the doors as houses, there needs to be a difference in the amount of blocks that can "see the sky" (i.e., have only [Air] or transparent blocks above them), counting five blocks in each direction (depending on the direction the door was placed). A house is initiated by a nearby villager (up to 16 blocks away horizontally along both axis, and up to 4 blocks vertically), and then either added to an existing or a new village. Sometimes the Villager AI doesn't behave as expected until the game is restarted.

Minecraft Wiki - Villager

So a normal house nearby an existing village with a door is enough to get villagers mating.