When you have the Open or Save dialog open in an application, you can type the following keyboard shortcut to toggle between showing and hiding the hidden files in a given directory:

Command-Shift-. (period)

If you have your Mac adjusted for a region where the comma is the decimal separator of fraction numbers, or if you are using a keyboard with a comma instead a point on the numerical keyboard, then the shortcut is

Command-Shift-, (comma)

This has to be the comma on the numerical keyboard.

[robg adds: This does not work in 10.5, and is one of the more-useful undisclosed features I've yet heard of in Snow Leopard.]