Not sure if iMessage is down for just you, or if it's everyone? Or maybe you want to know if iCloud's Photo Stream service or Siri is working right now. No problem. Apple's System Status Web page lists the current status of its online services and stores so you can tell at a glance what should and shouldn't be functional right now.
According to Apple, all systems are functional.
Apple's System Status page is tucked away in the company's online support site. It shows the current status for services such as iMessage, Siri, FaceTime, and Maps, whether or not the App Store, iTunes Store, and iBookstore are online, and whether or not iCloud services such as Find My iPhone, iTunes Match, Documents in the Cloud, and iWork for iCloud.
When I last checked, the page showed all of Apple's online services were working, which is always a treat. It's also nice knowing when services like iMessage aren't working for me that I'm not alone. Misery loves company, and all that.