I am not sure what is available as far as a ceiling mounted fixture with motion built in, but if you find one, you will need to leave one of the 3way switches in the "on" position at all times. I would assume when you want to turn the light on manually it would be annoying because you probably have to flick the switch and move around until the motion sensor sees you. But it does kind of defeat the purpose to use manually. I know they have outdoor wall mounted fixtures like this, never seen a ceiling mounted one. You can also run another 14/3 wire out of one of the switch boxes and put an additional 3way switch where you want and have two 3way switches and one 4way switch in the middle. Another problem you might find is finding a motion switch that will work on a a 3way or 4way setup.

Go to an electric supply house and ask for a ceiling mounted occupancy sensor. You'll need to intercept the wire going directly to the light not the 3 wire. You can either wire the switches to bypass the 3-way system or leave them alone and just leave them on and don't touch them. I think wattstopper makes one for the ceiling, you will only find it at a supply house, probably not to cheap though. They work on sound and motion too.

Take a look at Lutron's Radio Powr Savr ceiling mounted occupancy sensor that can be combined with a Maestro wireless switch.

That's a good choice too if you want to use the switch. Wattstopper is more for commercial use Lurton is more geared toward residential but they can be used for either.