It looks like the Mac launchd launchctl StartInterval is broken in Mac OS X 10.6.x, at least the current version (10.6.7). When I run a Mac launchctl script like this, which used to work just fine:
Mac OS X launchd plist FAQ: Can you share some Mac OS X launchd/launchctl plist file format examples?
I started to write a long tutorial on the Mac OS X launchd plist format, but I think an easier way (for me) is to point you to a couple of references that explain the launchd plist format fairly well.
Mac OS X launchd plist FAQ: Can you share some examples of how to use the Mac OS X launchd plist StartInterval and StartCalendarInterval keys?
Here's a quick look at the Mac OS X launchd StartInterval and StartCalendarInterval keys. These two keys can be used to represent time values in a Mac launchd plist file, and many people have questions about them.
Mac OS X launchd plist StartInterval examples
The general syntax for the Mac OS X launchd plist StartInterval key is shown here: