Mac OS X service runs as nobody rather than specified user

Hello, On a Mac running OS X 10.6 Snow Leopard I have a file org.apache.tomcat.plist in /Library/LaunchDaemons. It is owned by root. When I load the plist using launchctl the program runs as user "nobody" rather than user "_tomcat" as specified. What can I do to have the program run as _tomcat?
Thank you.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
        <key>Disabled</key>
        <false/>
        <key>Label</key>
        <string>org.apache.tomcat</string>
        <key>ProgramArguments</key>
        <array>
                <string>/Apps/Tomcat/bin/catalina.sh</string>
                <string>run</string>
        </array>
    <key>ServiceDescription</key>
    <string>Tomcat Server</string>
    <key>UserName</key>
    <string>_tomcat</string>
        <key>RunAtLoad</key>
        <true/>
</dict>
</plist>

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