Update 10:12 a.m. EST: Mission Control observed an issue with the second stage thrust vector control system and has scrubbed today’s launch attempt. There will be updates at noon and for now, the mission has been rescheduled for 9:38 a.m. EST tomorrow. Elon Musk tweeted SpaceX assessment of the problem:
Source: Gizmodo – How to Watch Today’s Historic SpaceX Rocket Launch [UPDATED]