One of the features in Mac OS X is the ability to load bundle code into arbitrary applications using the InputManagers directory. Basically, any bundle that is loaded into the InputManagers directory in a user's Library directory (or the root Library directory) will be loaded into every application that the user opens.
The InputManagers directory on OS X gives the user the ability to load any bundle into any application. The Objective-C runtime environment gives code the ability to dynamically change the mapping of any function at runtime. The combination of these two allows a bundle to modify the behavior of any application launched by a user. This fact is nothing new -- people have been discussing this for a while, and other people have been using this functionality to write neat software that modifies other software.
