I've been using a hat-mounted P4 for about a year now and have experimented with a lot of different kind of hat-mounts. I've got mine positioned so it's in my upper-right periferal vision. This doesn't give the overlay effect that you get from the glasses, but it does look less obtrusive and lets you make eye contact with both eyes. I can wear the hat-mount all day (and often do), while I couldn't wear the glasses over my eyes for too long without going crazy. The hat also absorbes a lot of the vibration, which will make it quieter for you though probably not for others.
The most important part of hatmounts is choosing a hat that can support the wieght. My first hat was a top-hat, with the PE attached to the brim of the hat. Because the felt brim couldn't support the PE, I had to put an L-shaped aluminum shim between the hat and my forehead, which extended a ledge out under the brim that the PE was then clipped onto using a notebook-binder-label clip. The shim transmitted the vibrations straight to my head, which gave me headaches until I put a thin layer of foam rubber on it to damp the vibrations.
My current hat (and the kind I reccomend) is a cap, kindof like the british cabbie caps. Mine's made by Kangol. These have a firm plastic brim imbedded in the cloth, and can hold the PE on its own.
I silicon-glued velcro to the PE and sewed felt to the underside of the hat, so it's quick to move the PE around. There is also a pencil sewed under the back of the velcro-felt on the brim, which forces the PE to point DOWN, so you can see into it when it's above eye-level. This way I look up and to the side to read, but can make normal bifocal eye-contact even to the side as long as the person I'm talking to isn't too much taller than I am.
You want another piece of velcro, snap, or something in back to keep the cable out of your face. This'll also help with weight ballencing. I actually keep another weight in the back of the hat to ballence it, because without it the hat will tend to slip over my eyes.
There's a sweet-spot for small displays. Move even a little bit outside of that sweet-spot and you can't read the entire screen. If you're doing something like glasses you can just position it right once, and they'll always be in the right place. With a hat you'll never have it exactly right (the plastic brim will warp over the months), so I reccomend a mounting that you can adjust easilly. Velcro has worked great for me.
With the hat you can position it so you have it right over the eye (overlay effect) or off and to the side. I prefer the latter mounting scheme, but you can switch between them just by moving the hat.
The firm plastic brim I have has warped over the months. You might be able to find a better brim. A baseball cap might even work, though I haven't tried it because I don't like how it looks on me.
-Brad Rhodes