Debian, and assuming Ubuntu and other deriatives.

doublebeta@Unbihexium-316:~$ ls -l /dev/ttyS*
crw-rw---- 1 root dialout 4, 64 Sep 28  2011 /dev/ttyS0
crw-rw---- 1 root dialout 4, 65 Sep 28  2011 /dev/ttyS1
crw-rw---- 1 root dialout 4, 66 Sep 28  2011 /dev/ttyS2
crw-rw---- 1 root dialout 4, 67 Sep 28  2011 /dev/ttyS3