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 crontab FAQ: How do I run a Unix job (or shell script) through the OS X crontab facility? I keep trying to edit my Mac crontab file, but my Mac won't save my crontab changes, or run my program.
Mac OS X - crontab, launchd, and launchctl
As of this writing (updated in 2014), the Mac crontab command seems to be deprecated on Mac OS X, and the Apple documentation encourages you to use their "Mac launchd" facility. Here's a blurb from the Mac OS X crontab man page:
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:
Mac OS X launchd FAQ: Can you share some Mac launchd examples (also written as launchd plist examples, or launchctl examples)?
In an earlier tutorial (Mac OS X startup jobs with crontab, launchctl, and launchd) I demonstrated how to use the Mac OS X launchd facility instead of cron to run what would normally be a cron (crontab) job. As I started working with launchd and launchctl, I realized it would probably be helpful to see several different launchd examples, specifically launchd plist file examples.