People have been doing this for years already with tools like Wireshark. The only thing the app he has released does it to draw a massive amount of attention to the already existing problem. I say superb. Brilliant effort. Well done. Hopefully more people will stop stupidly sending session cookies over unsecured channels now.
Also. If you want to use Facebook completely over https. Install the "HTTPS Everywhere" Firefox addon. It forces a number of sites to make all of their requests over an SSL secured channel.
