No go: Falcon 9 aborted its launch tonight at 4:55 am (Eastern) at T minus zero seconds.
Just after ignition there was a shutdown. They were able to make the vehicle safe, and are trying to figure out what happened. It appears there was a high pressure reading in one engine.
The next opportunity to launch is three days hence, though whether they will go for it then depends on whether they can figure out what caused the shutdown.
Update: More details here.