Does too. Looking up a hostname can take time, which will be wasted if you turn out not to care. And duplicate lookups will be cached.
You know, you have a good point there. I recently reverted some custom work I did on the RBL plugin that tried to cache positive results; it didn't seem that necessary as DNS can do that for us anyway. I hadn't thought of this in the same way. I don't think this patch could be considered for inclusion :)
-Jared
