Glen -
The "X" setting on conventional cameras selects the fastest shutter speed which would allow synchronization with electronic flash units. Although I can't find any specific reference to an "X" synch shutter speed in the D30 manual, the information on page 91 makes it pretty clear that 1/200 is the fastest speed that can be used with flash. It's important to note that flash units will synchronize just fine at speeds slower that this; synch problems only occur when you try to use a faster shutter speed.
I normally set the shutter to 1/60 for studio flash situations, in the manual mode, of course. The D30 selects its own shutter speed when used in automatic modes with EX flash units, but I like to use these units in the manual mode as well, so that I can use a slower shutter speed (1/15, for example) and let some of the ambient room light contribute to the exposure.
Anyway, 1/200 appears to be the D30's X synch setting.