email the user when an item is back in stock

I'm sure you've heard this one before but it would be really cool if, instead of a message reading "out of stock" when the inventory reaches 0, the user could click a link that subscribed him to a notification service that emailed him when the inventory was replenished. A bit like the Notify module but emails are only sent out when the inventory level is changed to a positive number and users can subscribe on a node-by-node basis.