Some Windows Phone 7 users may be receiving an update today for their phone that Microsoft says paves the way for the update that will add cut, copy, and paste, but at the same time these updates are going out, a new bug is appearing. Several people have posted on Microsoft Answers reporting that contact live tiles are disappearing from the start screen on their phones. I too have just had this happen on my HD 7.
I picked up my HD 7 and scrolled through the start screen and found the gap that you see in the screenshot where a contact live tile previously existed. The contact that I had in that location is linked to Facebook, Google, and Windows Live, and it appears from reading the posts on the Answers forum that the problem might be associated with Facebook. Perhaps Facebook has made some change that is causing this problem, or there might be an actual bug in Windows Phone 7. So far I have not seen a response from Microsoft about the problem.