Apple's yearly developer conference should see iOS 8, OS X 10.10, and likely some hardware.
Some Experiencing Delays and Failures When Sending iMessages [Update: Fixed]
Users are experiencing difficulties with delayed messages, messages completely failing to send, as well as read receipt notification errors. The problem is intermittent and not universal, but given the high-profile that Apple places on the service -- it's enabled by default on every iOS device -- any sort of outage is going to affect many users.
We've emailed Apple for comment and a timeline for a resolution, but have yet to hear back.
Update: iMessage appears to be working again for many.