I'm no expert on this by a long shot bt udev is suposed to be using the info in your sysfs
you got sysfs mounted in /etc/fstab ?
sysfs /sys sysfs defaults 0 0
this should result in the kernel making a directory
it should hhave a file in it named dev with the major and minor device numbers
should be able to type
udevinfo -q path -n ttyS0
perhaps we can go from here and diagnose further