I've investigated making IC a facebook app in the past, as have others, but one large obstacle is the use of inline frames.
Facebook apps are actually just an inline frame on Facebook with contents from the apps website displayed inside. The problem is IC already uses iframes for the sidebar. Bad things happen when one tries to stick IC's iframe inside of facebook's inline frame.
Rewriting IC's code to not use frames is a very big task. We'd need two copies of code to display a page, one for a normal website and another for display on facebook. Because IC has grown over a long period, it was never written with a specific architecture. To make the code maintainable, we'd have to write the code using something like MVC. With that we could have one view for the normal webpage, and another view for a facebook app while using the same model and controller for both. This isn't impossible, and it will happen eventually, but separating ICs code into an MVC architecture and then writing two seperate views is a lot of work.