It seems that when users try to send a message via iMessage the system will hang, then either fail, or send the message via SMS if possible. No word on why this is happening, but it definitely looks like an issue on Apple's side, so nothing users can do will fix the issue.
We'll keep watching and let you know when things get sorted out.
**Update** At around 6:15PM EST the iMessage service was restored, and appears to be working on all platforms. Still no word on what caused the outage.
source: The Verge