This icon normally represents a reminder notification for a task or an appointment... In most cases it should disappear as soon as you acknowledge the pop-up reminder by selecting "Dismiss", but I have seen it get "stuck" sometimes.
Check your Calendar and Tasks application and make sure that you don't have any unacknowledged alarms. If you still can't get rid of the icon, try resetting the device by holding
+
+
at the same time.