In 10.6 you can let Mac OS set the time zone automatically. Open the Time & Date System Preferences panel, and select the "Set time zone automatically by location" option.
This works using CoreLocation, just like the iPhone. Even though it's just an option, I thought I'd post it because people probably don't use the time zone settings very often.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2009083013552497