My intent is that unlimited would be basically unlimited, but I suppose if things become crazy I might have to put a cap in place (1000 URLs or something). The API already exists in a sense; check out the bookmarklet.
I've often wanted something that'll alert me when URLs change, and show the resulting diff (e.g. changes to the terms of service or privacy policy of the sites I use, or the page for tracking some FedEx parcel I shipped, or areweatwarwithiran.com). ChangeFeed is designed to basically turn any page into a completely impartial RSS feed.