Did you pick a specific model, yet? Projectorcentral has calculators that show you where it has to be mounted, both height relative to and distance from the screen. Each model is different. My Toshiba MT700 has to be exactly at the top or bottom edge of the screen. The Sanyo Z4 can be anywhere from 10% (of screen height) above to 10% below the screen. (I think it's 10%, close to that, anyway)

There are 2 kinds of shift that allow the PJ to be at different heights: optical shift and digital keystoning. Optical shift means the lens physically moves to change the location of the picture. Keystone means the PJ digitally calculates a different shape to adjust for the angle. This means it is actually projecting a trapezoid instead of a rectangle, but when it hits your screen it looks like a rectangle.

The best choice is none of these, but optical is far superior to keystoning, which will inherently damage the picture quality. My examples in the first paragraph are referencing optical shift only, there may be additional leeway if those PJs have keystoning. I refuse to use keystoning, so I don't consider it. I don't even know if my own PJ has vertical keystone. I think it has horizontal.

Yeh I've decided to get the infocus 4805.

Depends on the projector's distance from the screen. The offset of the 4805 from the center of the projector's lense is about 28% of the screen height.

At 13-14' feet from the screen you'll be able to fill a 7' screen (47" high). The offset from the center of the projector's lense at that distance will be 13" so you can mount the top of the reflective surface of the screen so that it's just over a foot under the center of the projector's lense.

You really don't want to be using the keystone adjustment on the projector, it negatively affects the image. You could however angle the screen a bit so the image hits it square on. I don't know how much you'd have to do that in your case and whether it would affect viewing.

I have very high ceilings as well so I'm going to be mounting my projector on the wall. I don't think it's a good idea to have the screen as high as it sounds like yours would be, especially at that short a distance from the viewers.

I disagree about having a high screen. In my experience higher screens make for even more awe inspiring viewing. It's partly what separates them from TVs. I'll see when the time comes though. Nothing's in concrete yet and there's plenty of room for adjustments and the screen could be far lower than the height I have in mind.

Wall mounting the projector sounds like an extremely good ide though. For some reason that passed me by. Probably because all the mounts are advertised as ceiling mounts. I'll definitel consider that. It'll be far easier to put up for a start.

A high screen looks cool when you stand in the back and reminds one of a movie theater. All the Ultimates around me have them set up this way for effect. But proper viewing when you sit down and actually watch something should be at least partly below eye level, say with eyes about 1/3 up the height of the screen. Helps your neck and other ergonomic concerns, plus probably matches the audio placement better if you have surround sound. The people who throw a plasma 7' up above a fireplace are nuts, IMO. (note that theaters now have stadium seating, putting your eyes as high as the screen, too)

All that said, my eyes are just at the bottom of my screen, a few inches up. Necessitated by placement, also. So I don't quite fit into the rule, either.

Oh, and a note about wall mounting. With it up high, you still need it upside down for most PJs, the 4805 would need this. Most people build a little shelf and then "ceiling" mount it to the underside. There are some mounts that can be used directly on the wall, but not too many.

Right. Unless you want your screen really high.

Spiky is right, follow the 1/3 line-of-sight to screen height or neck will kill you.

If you want a 4805-specific mount that is flexible for both ceiling and wall mounting, look up the "mandarax" one in the avsforums.