Without a doubt, the best way to protect your BlackBerry's display without any damage or loss of visibility, is a Martin Fields overlay.
I've used them for about a year now on various devices, and recently decided to get another order together for my P910a and BlackBerry 7290 and 7100t.
The problem is this; your display is important. It can get smudged, scratched, and otherwise molested. The 7200-series devices aren't so much at risk because the screen is recessed and it has a heavy matte finish to it, but regardless - the chance is still there that you'll drop it, scratch it, or do something to drive you insane.
I purchased my Overlays directly from http://www.overlayplus.com/
Now some overlays for screens lower the visibility of the device. They end up looking a little murky - or worse - and the best thing about the Martin Fields overlays is that, if anything, they INCREASE the visibility of the display they're applied to. These are, without a doubt, the best overlays for your 'Berry.
Note: the 7100t is not photographed yet because it is in the possesion of my wife and she's not in the house at the moment so I'll try to add pictures of that later. It is important to note that since the 7100 has a display that is right on top of the device, it is very likely that it will get scratched and banged up. Those big bright reflective displays show faults more readily than the traditional BlackBerry devices.
Installing these is a snap, they are sticky on the back side and easily can be dropped into place.
If you make a mistake or get dust underneath it, fear not! You can use scotch tape to pick up any stray fibers or dust particles. And the Martin Fields is great because you can actually wash them and reuse them - more than making up for the slight premium you pay to get 'em.
The end result is that on the 7290 you end up with a slightly reflective display, which is still highly visible outdoors!
I find that it makes the backlight more useful and brings a little more focus to the display. Usually using a screen overlay will make the display slightly less useful, but in my experience these make it more visible somehow. Go figure.
I also got one for my P910a:
The results are similar with the 7100t since they have a similar display. The overlay keeps it bright, and highly visible, while giving a nice protective coat as well.
Perhaps we can convince our intrepid sponsers here to start carrying these, they are absolutely the best overlays you can buy for your BlackBerry. I'll try to add photographs of the 7100t with the Martin Fields overlay later today or tomorrow.