I am interested in writing a new lock-unlock plugin for Apple iOS 4.x platforms. I am wondering if there is a plug-in architecture that supports developers implementing a replacement to the standard 4-digit pin code supplied by Apple.

I am guessing you would have to go the Cydia/Rock route and market to jailbroken-only iDevices.

I believe there are already some out there, including ones that mimic... hmmm... droid maybe (where you do a pattern with your finger).