So, do you buy a 16:10 screen (you can get those made custom), or go with the traditional 16:9 screen. I expect the vast majority will go with 16:9. That means that when you set up your projector to fill the 16:9 area, you will have some overshoot above and below the screen. If you have 100" diagonal screen, the usable surface is just shy of 50" tall. The overshoot at the top and bottom works out to a little more than 1.6 inches at the top and at the bottom. This normally means that the unused area (when watching DVD or movies will throw some dark gray light that will hit the mask around your projection screen. This should be barely detectable assuming your screen has a nice (typical) matte black border around the viewing surface. Note if you are buying a pull down screen (they normally have smaller borders, you might want to make sure it has at least 1.75" of black material at the bottom.
Of course, if you do go this route (a 16:9 screen), and you do project an XGA computer signal, you will have the first 24 pixels hitting the border at the top, and the bottom 24 pixels hitting the bottom border. If your projector is mounted (or placed) where you can access it, you could, of course zoom out just a touch to allow the whole XGA image to fill the screen.
I watched the HD73 on both my Firehawk (128") and the Carada Brilliant White (106"), and never noticed the light on the border, unless I was specifically looking for it.
Like most DLP home theater projectors, there are also some unused pixels on the left and right, so if you are trying to mount your projector exactly at the inner limit defined by the zoom lens, you may have a little space on each side of the screen. Optoma's lens throw info is further down this page.
Of course another big- for some, advantage of the Optoma using the 16:10 aspect ratio, is that this projector will double beautifully as a business projector. I guess that means a lot of people will be writing them off as a business expense.
Optoma HD73 Lens Shift and Lens Throw
To fill a 100" diagonal 16:9 screen, the front of the lens of the HD73 can be placed as close as 11.48 feet, or as far back as 13.81feet. (Based on Optoma provided specs.)
You can find a chart on the HD73 brochure on the Optoma website for seeing what size screen will work from any given distance. There is also a lens throw calculator that I haven't played with.
The HD73 has a lot of lens offset, which is the projector's fixed lens shift. The projector, if ceiling mounted, will have its center of lens, above the top of your screen surface. Conversely, if you place it on a table, the center of lens needs to be below the bottom of the screen surface. The amount of this offset is more than found on most projectors so you may have a problem if you are converting, say, a basement where you only have 7.5 feet tot he ceiling, and a fairly large screen, because the projector will be well above the top of the screen. If you use a flush type mount + the distance to the center of the lens, you will probably still be at least 7-8 inches from the ceiling.
I should note that most projectors with (variable) lens shift, will let you place the projector almost anywhere vertically between the top and bottom of the screen, some will even allow offsets that will allow the projector to go significantly above (or below) the screen surface. Competing LCD projectors usually have lens shift with lots of range. This is one of the weaknesses of DLP projectors in this price range.
DLP projectors like the Optoma HD73 (and HD72, HD70) and the BenQ PE7700, as well as the InFocus IN76 and IN78, Planar 7060, Mitsubishi HD1000U and HC3000, all lack lens shift. The offset can vary a lot. Whereas the Optomas', InFocus'and Mitsubishis' mount well above top of screen, the BenQ, by comparison, has a 0 degree offset, which puts its center of lens even with top of the screen. It's all just a matter of what will work for you, in your room. The Planar falls somewhere inbetween.
According to the manual the lens offset is 6.52 degrees to the top of the image, and that assumes you are using the full 768 vertical pixels, whereas, I believe most people will use a 16:9 screen and only 720 pixels of height. Depending on the zoom setting, for a 100" screen, I measured projecting the 100" diagonal from 13.8 ft - (the greatest distance for that sized screen) the offset was approximately 15.3 inches from center of lens to bottom of the screen. BUT that would be for the 16:10 image. So add an additional 1.6 inches to the bottom of a 720 line image. End result, you are looking at having the center of lens 16.9 inches below (bottom of screen) or 16.9 inches above (top of screen) for a 100" screen at max distance of 13.8 feet. (Please note, these were quick measurements - do not rely on the them for precision.)