From the NYTimes Bits blog:
Apple on Sunday released a web tool to address a problem that has affected some iMessage users: When they switched to a non-Apple smartphone, like an Android device, they could no longer receive messages from iPhones.
When someone with an iPhone switched to a different smartphone, like an Android phone, the phone number would remain attached to iMessage, which is usable only on Apple devices. So when iPhone customers tried to send text messages to that number, sometimes those messages would never make their way to the intended recipient.
Apple’s new tool, which many spotted on the web over the weekend, allows a former iPhone user to enter a phone number to detach it from iMessage. When a phone number is entered, a text message returns with a confirmation code, which also must be entered into the web tool.