When Nikon first introduced the D1, people were having problems getting consistent flash exposures using the SB-28. The problem is reportedly with the reflective quality of the CCD imaging chip that doesn't work as film does with the TTL sensors.
Nikon introduced the SB-28dx not too long afterward. The dx addition allows the D1 to do the flash TTL metering using the monitor preflash lighting. For the original D1, the dx version flash is the only one that will provide reasonable TTL operation. I assume this is true of the D1X and D1H as well. Flash performance with the new D1 cameras is supposed to be substantially improved as reported by Moose Peterson.