If you want a file to act as a template, or stationery, turn on the Stationery Pad option by choosing the file in the Finder and selecting Edit -> Get Info and clicking the Stationery Pad checkbox. When you open a file with this option turned on, a new, untitled copy of the file is opened; the original file is left untouched.
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If you want to change the original, deselect the Stationery Pad checkbox.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040430140546864