I've heard various ideas such as limiting it only after mods have expended their points (this will require implementing a cooldown to prevent a user from getting points again too soon). I want to hear your feedback, and I'll roll together something for the next major update of the site. Leave your comments
This is an interesting idea, have a few (1-3) "reserve" mod points floating around for high-karma users that *could* replenish after a month or so.
What about connecting the replenishing to the upmoderation of your own posts? For example, if you got your post upmodeed (and are not on your upper limit of permanent moderation points), you get a certain fraction of a point (say, 0.2 points) into your permanent pool. Of course you can only spend full points (it may also make sense to only display full points, i.e. rounding down on display). Similarly, downmods of your own posts may reduce the number of permanent mod points you have by the same amount.