Chris, Nikon couldn't figure out how to get reliable off-the-ccd metering with the original D1 camera. People trying to use TTL mode got very inconsistent results. Nikon came up with a workaround and introduced the SB-28 DX to provide a solution. In DX TTL metering, the flash exposure is metered during a preflash cycle (some super-rapid pulses before the shutter actually trips). The metering is done with light as it enters the camera rather than as it reflects off the CCD. This workaround works, but it is a step backward from "live" TTL where the the exposure can be modified as light changes during the actual exposure. And needless to say, this leaves Nikon DSLR users unable to use the existing Speelight system without taking several steps backward and using flash-controlled auto or manual modes.
The DX feature is NOT designed for all digital cameras, just Nikon's DSLR bodies. It is not needed for Coolpix cameras, nor is it needed for Kodak and Fuji cameras built from Nikon bodies. Both Fuji and Kodak provide off-the-chip metering and work with existing Nikon and Nikon compatible Speedlights. This issue was one of the most important reasons I went with a Fuji S2 instead of the D100. I didn't want to take a step backward in flash technology while having to buy new Speedlights to replace the two Nikon units I own.
From Nikon's standpoint, the DX thing is now a marketing plus. It also means that third party flashes with Nikon emulation won't provide TTL metering with Nikon's DSLR cameras. If you want 3D-TTL, with your D100 you have to buy a DX speedlight. Since the SB-28dx is being phased out, that gives you the SB-80dx and sb-50dx to choose from. Also note that wireless TTL (from the SU-4 and the SB-50DX) capability is compromised by the DX feature and Nikon warns against trying to use wireless TTL with its DSLR bodies.
Yes, you can work around the issues using flash auto and relying on your postview to check your settings. But I think it's time for Nikon to quit marketing a kludge as a feature and have Kodak or Fuji tell them how to meter off the chip.