Older versions of K4PC would allow you to double-click a non-drmed mobi and read it without being registered (I used to do it), but I just tried it with my newer version and that no longer seems to be an option. Unregistered = unusable apparently
Sorry, I was mistaken... if you click on the teeny, tiny, almost unnoticeable "Continue without Registering" link, it won't prompt you to register anymore. Allowing you to read personal non-drmed mobi's at your leisure.
