Update: Continuing independent testing shows that iMessages are now being automatically switched to SMS texts.
Update 2: Users are reporting that iMessage services have been restored some three hours after the outage was first noticed.
AppleInsider has received a number of reports noting that Apple's iMessage is not functioning, and has independently confirmed that the messaging service is down.
Both iOS and OS X clients are unable to send or receive messages sent through Apple's service, with users seeing a perpetual "Sending" prompt. Some members of the Apple Support Communities forum system status webpage, all services are online.