Of course everybody is going to say you shouldn't do it. Yes it's very dangerous, no you shouldn't be doing it regularly, but I will admit I have thought about this also, and I bet a lot of you have also thought about it too. And I think it's very possible to do this. Not rapelling off paracord specifically, but the smallest setup you could rappel off of for a one time emergency use. Fortunately, someone else also thought about it came out with this

http://www.uscav.com/productinfo.aspx?productid=7342&tabID=1287

I don't know the specifics, but during some free time I have looked into setting up something like this just for fun. I know there is a new 5mm cord on the market that is rated at 5000 lbs, so the strength factor is high enough to say that it is definitely possible. I did see another one with a descender specific for that type of cord, it was more of a rectangular shaped piece with holes, and had extra bends for increased friction. Even so, I'd wear gloves, because small cord can cause burns very quickly.