Considering it still loads it correctly with the world file gone out of InstalledWorlds, are you certain you haven't put the world file in the Program Files\Electronic Arts\The Sims 3\GameData\Shared\NonPackaged\Worlds location as well?
Thanks, that was it. It was a world I installed quite a bit ago, so I don't remember doing that, but it was there. And yeah, I was figuring my next step was taking out the cache, but this solved it.