FC4: su fails in init.d script

[Vasiliy Gagin]

One of my scripts in /etc/rc.d/init.d directory (jboss to be exact) is
using "su - jboss" and is failing during start up with message:

su: Permission denied

It works fine if I login as root and run it through "service jboss start".

I was under impression that during start up scripts in init.d are run as a root.
Any ideas why is it happenning?
Is it SELinux trick again? So SELinux if I'm blaming you for no reason.