I switched to a Nexus 5 recently. I deactivated iMessage, ensured my phone was no longer registered with Apple and asked friends to delete existing conversations.
It has been about a week and I still cannot receive text messages from iPhone users I've iMessaged in the past.
What is perhaps most frustrating is that Apple seems incapable of changing the situation. Adding insult to injury, Apple is requiring that I pay a $20 fee to speak with a technician.
This is false. I just called 1-800-MY-APPLE (details in my reply to GP) and they said I had already deactivated iMessage for the number and that I needed to tell all of my friends to delete their existing threads with me. And then even that did not fix the problem.