Can someone shed some light on this for me?

I'm trying to create a bootable CD on a Sparc IPX system under 
1.4.2_ALPHA.  I've created an ISO disk image file of the 1.4.2 tree using 
mkhybrid.  That works fine, and the resulting CD is mountable and usable 
with various systems, so I decided to try any make it bootable on the 
Sparc just like the Comdex CD.  I built mksunbootcd and run it specifying 
the ISO disk image file for the first argument and the syboot-142.fs file 
in the NetBSD-1.4.2 tree as the 2nd, 3rd, 4th and 5th arguments.  It 
appears to correctly write the Sun partition table - I've dumped the 
first block on both my new CD and on the Comdex CD and they appear 
similar.  But when I try to boot the new CD I get a "Data Exception" 
error from the boot ROM.  The Comdex CD boots fine but my new CD doesn't, 
so I know it's not the CD drive.

Does anyone here know what magic step I missed?  I'd appreciate any 
insight, hints, ideas, comments, etc.

Thanks,
-bob