Serial Terminal - Can't get getty to run via inittab

I'm trying to get a headless Fedora 6 box working with a serial terminal
on com1. I've got grub set up so that kernel output goes to the
terminal, and I've added this line to my inittab:

s0:345:respawn:/sbin/agetty -L 9600 ttyS0 vt100

but agetty is not being run.

If I run agetty with this same command line explicitly (from a prompt),
then the terminal gets a login prompt and I can login fine, so it seems
to just be a matter of getting init to run agetty.

I've read several how-tos on this but no joy. I've also tried mgetty
with the same results.

Any ideas?

TIA.