I believe the difference is down to how the two flashes work when you're in TTL mode: the DX produces a pre-flash that's used to determine the exposure by monitoring light reflected from the shutter curtains immediately before the shutter opens and the main flash fires.
If you used a standard SB-28 I'd guess TTL mode simply wouldn't work since the flash can't emit the required pre-flash - but you should still be able to use A mode where exposure is determined by the sensor on the front of the flash.
You can't use the SU-4 with the SB-28DX in TTL mode since the pre-flash would trigger it, and the manual also says "TTL multiple flash operation using a sync or remote cord such as TTL Remote Cord SC-17 or TTL Multi-Flash Sync Cords SC-18 or SC-19 is not possible".
For clarification: With digital cameras there is a longer shutter delay and therefore a different sync cue; that is taken care in the DX speedlight models and they will operate flawlessly on the film cameras as well. The non-DX speedlights may not properly function with a digital camera. Both the SB-28DX and SB-28 will work on a SU-4, the question is how not to pre-trigger them with the pre-flashes when in 3D Multi-sensored-Matrix-balanced fill flash. The way to do it is simply to set the triggering flash in standard TTL, (one icon only, showing) or tilt the head of the triggering flash. That is all. If someone wants a flash for a film camera, that will serve him later on a digital camera such as the soon to come D100, then it is better to get a DX speedlight, either the SB-28DX or the SB-80DX.
Thanks for the info. Strange I know I have read at least a dozen articles on the SB-28DX same with Nikon digital cameras, looked at Nikons' site and e-mailed them. This is the first answer to this question. Thank you.
Does anyone know if this a camera requirement or a flash requirement? Or asked in a different way... I notice Fuji claims their S2 Pro will be compatable with all flavors (ie including non dx) flashes... is that just marketing hype, or has fuji figured out something nikon hasn't... this is probably something that has been talked about and answered before... I've just missed it.
That's an interesting point. I wonder whether Fuji have found a way to do TTL flash metering with the non-DX units?
This doesn't seem to be the case for the Kodak DCS600 series cameras which are based on the F5. According to the SB-28DX manual the TTL auto flash modes aren't supported - Auto Aperture mode which is controlled by the sensor on the flash is used instead.
It may be Fuji are simply doing the same thing - relying on the A mode so it doesn't matter if you use a non-DX flash.