When I shoot film, I typically shoot at 1/60 given that I usually use ASA 100 film. My camera will synch my strobes at this shutter speed. Presumably, if one has to shoot at 1/250 to synch the strobes, I assume you would set the ISO to 400 when shooting digital.
The specification is for the fastest
speed at which sync can occur, not the only
speed at which sync can occur. The issue is how fast the shutter blades move across the sensor. At speeds >1/250, the shutter opening essentially forms a thin strip that moves across the sensor, the whole sensor is never exposed at one time. So your short duration strobe light will only fall on a part of the image.
There is no problem with syncing at speeds lower than this.
In the D1X, d70, d100 etc things are different. At speeds >1/250 the exposure is controlled by rapidly switching the sensor on and then off again, while the actual shutter remains open for 1/250th of a second. This means you can use very high shutter speeds to freeze sunbursts/sunrays, and yet can still get fill flash on a closer subject without shutter blades getting in the way.
Why would I take a perfectly good camera underwater??
D300, D200, D70, 12-24 f4 AFS DX, 60mm f2.8, 70-200 f2.8 AF-S VR, 105 f2.8 AF-S VR, Tokina Wunderlens. Photo galleries @ Ruaux.net