If, like me, you run a server somewhere pretending to be lots of servers thanks to Xen, and your guest domains keep logging problems with ttyS0 (like this, perhaps:)
May 5 06:30:10 my_guest init: ttyS0 main process (1391) terminated with status 1
May 5 06:30:10 my_guest init: ttyS0 main process ended, respawning
May 5 06:30:20 gamecity init: ttyS0 main process (1430) terminated with status 1
May 5 06:30:20 gamecity init: ttyS0 main process ended, respawning
That's right. TWO lines every 10 seconds. Grrr. Now, xen uses hvc0 and not ttyS0 for domU guests. So, I don't actually need ttyS0 unless I need my guests to have access to the physical serial port on the server. I don't. So, the easy fix is to stop ttyS0 from being created (ever). To do that, on my Ubuntu 8.04 guest, simply remove /etc/event.d/ttyS0. Don't remove this from Dom0, because that almost certainly will need access to the real serial port.