Seb Pauquet launches a request into the lazyweb to put some reputation filtering on top of del.icio.us. The overall concept isn't too difficult to implement, the tricky bit is not hammering del.icio.us and getting your service blocked. As far as I can tell, to get the full effect (using all of a tags history) you'd have to scrape a bunch of del.icio.us pages.
This just highlights the maddening point that Joshua zealously guards the implementation and data of del.icio.us. As far as the UI goes, I enjoy his staunch position. Keep It Simple Stupid. As far as the data and APIs go, let a thousand flowers bloom. The nominal reason to deny access has been server load, prior miscarriages of usage, dealing with crawlers, stability, etc. etc.
I wonder if we're to the point that if you're going to build a web application, work on the Web services API real early if not first. Plan on getting hammered by people building bad, broken, wrong, and way cool applications on top of it. Your elegantly designed frontend is just gravy.
A little more thinking out loud. How big a group do you need for a del.icio.us clone to be useful? Someone's gonna come along and do an easy to install opensource knockoff, with an interesting API and standardized data import/export. Throw in an inter-delicious protocol and you'll have the Wiki story all over again.
It could happen!!