High speed syncing

1/8000th f3.8 ISO 400
Two flashes at 1/8 power

1/2500th f3.8 ISO 400
Two flashes at 1/8 power

Here is the strobist article that describes the technique being done with high shutter speeds using flash. Nikon CLS and Canon E-TTL can perform these kinds of shots too using small flash pulses to cram as much light needed onto the subject in fast shutter speeds.