How high to hang nelson saucer?  

mine is 30" to the center of...
mine is 30" to the center of the lamp and it looks great!

Lower is better than too high
I find a lot of people hang their pendants too high. I prefer sitting down and having the underside of the fixture level with the top of my head. I have a Moooi Random Light over my dining table, and it's only 23" above the table top. A Nelson saucer would look better at a lower height than 30", especially when admiring it straight on from a distance.

I have long felt
the same. Similarly, I advocate for lower placement of wall art than is often practiced. The intention is to invite occupants to be seated -- to offer comfort, in other words. The oddly high placement of wall-mounted viewing screens is thus particularly painful to me. . .
In an extreme example, a friend decades ago placed an opaque conical fixture hanging from the ceiling on a cord, just a foot above the broad low coffee table in the living room. This was often the only light present in the room at night. I expect you can imagine the atmosphere this provided -- for better or worse.

I like the sound of that,...
I like the sound of that, the bulb type is really important too, there is no way I'd use one of the compact flouresecents over food, unless you were having a junky themed dinner, the halogens in the shape of tungstens are a bit better.
I like keeping lighting as low, warm and diffuse as possible and have a couple of spotlights, those ones on poles popular in the 70's are handy for that.

The chief lighting
innovation in my main space is concealed lighting behind low wall cabinets, which are themselves lifted off the floor in places. This allows warm low-level incandescent light to wash up the oyster-white wall and also spill out beneath the cabinets to rake the carpet, etc. Objects placed on the cabinet (it's a low credenza height) are thus silhouetted, and plant materials near the corners are as well. The effect is subliminally landscape-inspired, I guess, and the light on the wall makes its placement in space indeterminate, expanding the room. Very soothing -- I never tire of it.

SDR
Photos, please.

.
So many hanging fixtures just feel wrong. Including many in my
home. We have been gradually replacing some at a much better
height. Others, placed at odd heights, we will fix at some point.
Dining table? I like to be under it when sitting and looking over it
when standing. Like suggested, hang it lower than 30, 24-26?
I hung a corner coffee table pendant very low and have used a
rubber cord clip to try a few different heights over time. It actually
looks better much lower than i originally thought.
Look through some photo spreads. Very odd placements...
http://blog.modernica.net/?page_id=364

How high the moon?
Lower makes sense if hanging directly over a table, but pendants look dumb (and dangerous-- ouch!) hung low in the middle of a room, like a couple of the examples Rockland linked to-

WOW...
THREAD imitates life!! We were JUST having this..."conversation" this weekend ourselves since we had an electrician over to hang some fixtures and repair others. I like the low hanging look myself. I am even a fan of something I am seeing a lot of lately which is pendants hung over side tables with the diffuser at table lamp height. We did that this weekend next to our bed. We have one Kartell Bourgie table lamp on a "nightstand" and had a Kartell GE pendant hung on the other side of the bed with the shade at the same height of the Bourgie. But, now we need to revisit the pendant over the dining table...

32 inch above table
Got the nelson saucer light installed over the dining table now. 32 inches above the table top. It looks great but I think it will take a few weeks for me to get used to seeing it there to truly appreciate the light.
Now I want to order two more nelson bubble light for my living room. Our living room has vaulted ceiling and right now all the lights are on the periphery of the room so the center of the room is fairly dark. I don't know if I'd want something too low in the living room though since I like to rearrange our furniture.

Periphery lighting
with a less-well-lighted center could contribute to a visual expansion of the space. Eyes directed to the "horizon" will have that effect. A single pool of light on the rug, from a concealed down-light, preserves the space in the center of the room that would be compromised by a visible fixture hanging from the ceiling.
Lighted ceiling, with the color (white ? off-white ?) brought down to a band at picture-rail height, will also expand the space. The subconscious effect is of "sky." Wright used this effect often, from the Prairie period on. The lighting is either from concealed sources above/behind the band (Wright), or from up-lights (torchieres, 'twenties on). When cheap halogen torchieres came in in the 'eighties, America's living rooms experienced a revival of good lighting -- for $25 a pop.
I'm talking about the quality of light, not appearance of lamps. With hidden light sources, you don't have to consider whether the lamp looks the way you'd like. Cheap cans on the floor can produce the same effect as ugly, out-of-date torchieres. . .or the "wrong" Nelson saucer ?

Further:
We feel best, day or night, when not constrained. Since our ordinary mode of movement is horizontal and our most vital objectives are usually viewed ahead or behind, not above, the view -- either real or suggested -- in the horizontal plane is what we seek. If we cannot have a view from a window, a picture on the wall is the next best thing to giving us relief from a feeling of confinement.
Thus, any strategies for expanding our personal environment horizontally -- either physically or visually -- are worth exploring. The overhead expansion that Wright achieved by the method mentioned above, is a secondary way of providing relief from constraint. Either of these strategies, or a combination of the two, are worth achieving in some measure.
Since it is light, and not dark, that draws the eye -- with the contrast between the two providing enjoyable relief from monotony -- the light that our lamps and fixtures provide is a great deal more important than the hardware itself, to the pleasure of the spaces we are dealing with -- it could be argued. Unfortunately, it is far easier to choose a lamp than to imagine in advance what the effect of the light it sheds will be, when we get it home.

I was just looking at some Wr...
I was just looking at some Wright and Mackintosh interiors, I prefer the gentle light of Mackintosh, his muslin screens instead of curtains is pretty much what I'vé done, a spot light bouncing off it at night works well.
Does anyone else think the Mahogany thread needs to die?

You're right, SDR-
Most people obsess on the aesthetics of lamps and pay no mind to the way the room's actually lit, I suspect. You can get a lot more mileage from some cheap little can-lamps --strategically placed-- than an ill-situated fancy-pedigree fixture.
(Like Dieter sez: being well-balanced is better than being exalted!)