Apple's iMessage service is experiencing intermittent sending issues. This same issue cropped up previously back in September, with users reporting that the progress bar would sometimes hang, and issues with the service were reported again a few days ago. Apple issued a brief statement that it is looking into the problem and will provide more information as it becomes available.
According to Apple's iCloud Support page, "some" users experienced issues with iMessage and FaceTime this afternoon. Users reported that while attempting to send a message with iMessage, the progress bar would hang, causing the message to fail. Apple responded with the brief statement, "We are investigating and will update the status as more information becomes available."
iMessage is Apple's messaging system that allows iOS users (iPhone, iPod Touch, and iPad) to swap messages without the associated text messaging fees. iMessage is available with iOS 5 and later, and with OS X Mountain Lion 10.8+. Back in September, the problem followed an outage at Apple's iCloud service.
Currently, the Apple iCloud Support page says that the service is back up, but some users around the Internet are still reporting problems. It is possible this issue is due to superstorm Sandy, which has been wrecking havoc on the east coast. Gawker, The Huffington Post, and other websites and services have all experienced outages as a result of the storm.