Yeah, we've got some to sell now. And I figured out that you can kill all the animals in a plains chunk and generate horses in old chunks that way (got 1 successfully so far, then stopped).
You sure that works and you didn't just miss a lil booger? I suppose they changed horses respawning compared to other animals but I would have figured ever since one of the old updates nothing respawns on grass if you kill it like it used to. If that does work though that's pretty awesome actually. I've heard there is also a plugin that enables horses to spawn in even 1.5, and previous patch, plains biomes which is pretty awesome