I don't know of any distros offhand that autostart sshd and have a default username and password, but it should be fairly simple to make your own. The FreeBSD docs have a guide on doing it (if you don't mind installing Linux from a BSD shell), or if that doesn't suit you, simply mount the ISO and squash filesystem of a LiveCD on the loopback device(s). Copy the contents of the filesystem onto your hard drive, chroot into it, set the root password and make sure that sshd is set to autostart (and maybe verify that your network settings are correct). Then mksquashfs the modified filesystem to create a new squash image, then recreate a new bootable ISO with the modified squash image on it.
I perfer debian based distros but i may try and go the centos root and then use debootstrap and some grub trickery to install a debian system from within it, because im not sure how to modify a debian cd in this way.
Dont know if debbootstrap will work from centos though, but it should, ive used it before for modding an xbox
Ive got a LightsOut board for an old compaq/hp box somewhere, but i dont think it will work on the machine i want (its just a normal selfbuilt box)
In the end i what i did was customise an ubuntu disk so that it started SSH at boot, booted from it, then used it to create a debian install in another partition using debootstrap, chrooted into it, installed and configured SSH and changed grub accordingly. Then i prayed and rebooted into the new debian system.
Worked well. Still running.
P.S ive noticed the opensolaris live/install cd has a nice grub option to start SSH, in case anyone finds this thread.