An iPhone App launched about a week ago was able to do just that. Let you know when you've been defriended and by whome. The App Called Defriended created by i-Doodz has unfortunalty been blocked by Facebook. As it violates Facebook's Developers platform policies which states " you must not notify a user that someone has removed the user as a friend." In other words, Facebook doesn't want you to know who doesn't want to be your friend ".
Today, a message on the i-Doodz site states:
Last week our developer was bored one evening, had an idea, and a few hours later uploaded the "defriended" iPhone app. it unexpectedly got a lot of attention, then subsequently Facebook blocked the app, causing it to stop working. We will be looking at how feasible it is to make the app work without a corresponding facebook app, but in the meantime if you've bought the app you should ask apple for a refund - and please do...we don't really want to get money for an app you paid for and that doesn't work. We apologise for the inconvenience.
If you really want to know which of your friends have removed you as a friend, you are better off using the Facebook friend checker greasemonkey script which Facebook can't block. Any service which attempts to use Facebook's API for the purpose of notifying users about those friends who've defriended them are typically banned. Yes, being unfriended is extremely commonplace.