my former employer always talked about putting a long cork screw up the seat tube of a bike geared to the pedals you could engage when you park....all I could think about was what if you ever forgot to disengage it .

I don't think any of these things would do you any good in a major metropolitan area and it seems like most might just end in you hobbling your own ride. In NYC i lock it up or bring it with me and I don't even like locking it up on the street for more than a few hours.