Thoughtful, detailed coverage of the Mac, iPhone, and iPad, plus the TidBITS Content Network for Apple consultants.
 
Turn Off Filename Extension Warning

In Leopard, Apple fixed an annoying aspect of working with the Finder in Tiger. Previously, if you changed a file's extension, the Finder prompted for confirmation. But since no one has ever accidentally changed a filename extension, Apple thankfully added an option to turn that warning off in the Leopard Finder's preferences. Choose Finder > Preferences, and in the Advanced screen, deselect Show Warning Before Changing an Extension.

From Adam C. Engst