Update 8:24 p.m. ET After trying twice this evening, technical issues forced SpaceX to finally abort the launch. The crew started a half an hour countdown. About sixty seconds before launch, word came in over the radio the crew would be taking "the safe path," this evening. The second launch was off. As Musk explained on Twitter, nothing was wrong with the ship in the lead up to the second launch. It was a gut feeling thing: 

SpaceX's next attempt will be in "a few days," after the team inspects the rocket. 

Update 5:42 p.m. ET   Except things didn't go smoothly at all. The spaceship started the ignition process and was two seconds away from liftoff. But the ground computers realized something was amiss and the project was delayed, and possibly aborted, at the last minute.

There's still a chance the takeoff happens today. All is not lost. 

Musk discloses the problem: 

Original After some technical difficulties delayed its initial plans on Monday, Elon Musk's SpaceX program wanted to launch a Falcon 9 rocket Thursday evening, from Cape Canaveral in Florida, marking the first Thanksgiving space launch since 1956. It didn't go as planned.

Wired has a good recap of the problems suffered Monday's launch. Once Musk and co. realized Monday was no longer an option, they agreed with the FAA's suggestion to push the launch to Thursday to avoid complicating Thanksgiving travel plans further. The skies were already plenty full without a spaceship launch.

This is the first time SpaceX will attempt to launch a geostationary satellite. The Verge explains why this is hugely important for SpaceX

Should today's launch go smoothly, it'll mark a significant milestone both for SpaceX and for the commercial space transportation industry more broadly. The company's Falcon 9 will blast the SES-8 communications satellite — owned by Luxembourg-based SES World Skies — into geostationary orbit. It's the first time SpaceX has launched a commercial communications satellite, and will set the stage for the company to perform subsequent launches that currently take place overseas.