Actually, I'd say that Centaurs for horde is perfectly reasonable. As has been pointed out, the High Elves fought against both the Orcs and the Trolls, yet they've been accepted into the horde. The Forsaken were once the humans of Lorderon who fought against the Orcs, but they're now part of the horde. Just because there were conflicts in the past doesn't mean it's impossible for alliances to be formed in the future between two factions.

I think, if anything, the most likely source of redemption for the Centaur would be this fellow:
http://www.wowwiki.com/Centaur_Pariah

He seeks to unite the Centaur clans that are at war with each other in Desolace. He believes that the centaur should take pride in their origins and live up to their heritage rather than squabbling with each other over the barely-inhabitable lands of Desolace. If anyone could redeem the centaur clans of Desolace, it's him. Also take into account that two of the Desolace tribes are actually factions you can gain rep with in WoW currently. If they were united, it's possible that they would either become a new faction, or potentially a new playable race.

Besides, the Desolace centaur tribes are no where near Tauren lands, so it's unlikely that the conflicts with the Tauren involve those particular tribes, anyway.