Little known fact: LCD displays work by selectively polarizing light, and then passing it through a polarizing filter which is built into the display. If you remove the polarizing display, there is nothing to block the polarized light, and the entire display appears to be blank. (Bottom of picture)

If you then place a polarizing filter between the display and your eye (such as a pair of polarized eyeglasses) you can read the screen perfectly. (Top of picture). This provides some privacy from shoulder surfing when using your PDA in public areas such as mass transit rail systems or elevators. Obviously, you need to wear polarized glasses whenever you wish to use your PDA, and this method won't protect against an attacker also wearing polarized glasses or using a polarizing filter on a zoom lens.

Removing the polarizing filter from some displays (such as electronic calculators) is relatively easy, while in other displays (such as a Palm V) it requires a little more work, but is relatively easily accomplished by a determined tinkerer with a good selection of hand tools (screwdriver, X-acto, heat gun). For the Palm V, simply use the heat gun to open the case, unfold the display from behind the touch screen, and scrape off a corner of the polarizing film from the front of the display, then use that corner to gently pull the rest of the film off.