How to run a nightly backup job on Snow Leopard Server via launchd

Question

I have been trying to configure a nightly backup job on my Mac Mini server, and after a lot of Googling and digging and man-page'ing, I figured out that launchd seemed to be the correct too, and tried setting up my own plist fil. I've come up with this (stored in /Library/LaunchDaemons/dk.revealit.NightlyRSyncBackup.plist):

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
        <key>Label</key>
        <string>dk.revealit.NightlyRSyncBackup</string>
        <key>ProgramArguments</key>
        <array>
                <string>/var/root/run-rsync</string>
        </array>
        <key>LowPriorityIO</key>
        <true/>
        <key>Nice</key>
        <integer>1</integer>
        <key>WorkingDirectory</key>
        <string>/var/root</string>
        <key>RunAtLoad</key>
        <false/>
        <key>StartCalendarInterval</key>
        <dict>
                <key>Hour</key>
                <integer>3</integer>
                <key>Minute</key>
                <integer>15</integer>
        </dict>
</dict>
</plist>

Problem is, it doesn't work. I can load it with launchctl, and it appears on launchctl list with exit status 0. But if I check my backup files, they are not updated.

Any clues as to what I'm doing wrong (assuming my backup script works as it should)?