I have a script that I want to run at boot time. This script needs to 'su' to a specific userid in order to start an application. If I run the script from a command line, (as root) it works. If it runs at boot time, it fails.

From what I can figure, the su command is failing.
Are scripts, run at boot time, run with root authority? If not, how do I get the su command to work at boot time.

Sorry ... I guess I should add that the Linux OS is Red Hat Enterprise Linux V4



Would having the script run from inittab make a difference?

Yes, the startup scripts are run as root. How are you doing the su command? In init scripts I usually do:

su user1 -c 'command to run as user1'

If it is owned by root,root, and executable, it should and will run at bootup. For example, all the rc.d scripts are root-owned, and are executable, and root is who boots the machine.
You shouldn't need su/sudo, unless you wanted to change to a lesser user for some reason.