In December I experienced a slew of problems related to using my legacy email@example.com email address. Find my iPhone stopped working and mounting my iDisk and MobileMe became erratic. I could find nothing on the net suggesting there was a problem — let alone addressing it. Long story short, I had to change all my logins, on my mac and on my iPhones to firstname.lastname@example.org (the fully qualified email address, not just my account name) to resolve the problems.
Possibly answers the questions “Why did Find my iPhone stop working?”