Luckily I tried to ssh into it, and the sheevaplug password worked.
Maybe the event.d/serial is broken only in that
rootfs-f10-sheevaplug.tar.bz2.
The event.d/ttyS0 was added in the rootfs-f10-sheevaplug.tar.bz2 to support the default
kernel (patched 2.6.22.8, IIRC). If you are using the standard kernel, you need to remove
/etc/event.d/ttyS0. This is mentioned as a note on
http://openplug.org/plugwiki/index.php/Fedora-ARM-10_Plug_Computer_Distri...
HTH.
Thanks,
Amey