A320 Captain reports a citation jet taxiing between his aircraft and the terminal building after pushback and engine start at JAX.

Narrative

We had just completed engine start after pushback from the gate at JAX. I started to release the parking brakes as the First Officer called JAX ground control for taxi instructions when a Cessna Citation taxied across our nose from right to left between us and the terminal building. The Citation had less than 10 feet wingtip clearance from the nose of our aircraft and a similar spacing from his right wingtip and a ground vehicle. If we had moved forward at all; we probably would have had a collision. Ground control had said nothing to us about the Citation taxiing in front of us. The airport information page says specifically that taxiing aircraft are to remain on taxi centerlines on the air carrier ramp.Ground control was more interested in moving aircraft around than safety. If I had released the brakes a second or 2 sooner; there would have been a collision. Ground control needs to follow its own procedures and make aircraft hold position until it is safe to taxi. The Citation should never have accepted taxiing so close to our aircraft not knowing when we were going to start moving.