There is a newer version of the iMac firmware included with Mac OS X Server. It is installed as part of the NetBoot Server software and can be found in the "Admin" folder on the Mac OS X Extended format volume the NetBoot software was installed on.
There are two ways to tell the different firmware updates apart. The version of the updater application included with the NetBoot Server is 1.1. The updater application from the web site is version 1.0. Also, the modification date for the "iMac Firmware 3.0" file is December 1998 for the one on the web site and February 1999 for the one installed with the NetBoot Server.