I just last night found out one of my images was used as a book cover...
I was browsing the web using a very cool tool called TinEye (you just feed it a photo and it gives you back all occurrences of that photo in their database) and found this book cover.
I am now in the process of finding out where they obtained their license as I cannot remember to have seen any payment for this yet.
PICTURE CREDIT Arthur Gebuys - Gebuys.com
update 2008-07-25: the payment for this usage is settled!