I can see three possibilities.
1) The serial port is one way. (I hope this isn't it....)
2) I'm setting something up wrong. (Entirely possible)
3) The serial port is set up as one way by the kernel.
I tend to think it's option 3 as when I rebooted my Z while the serial cable was connected I got kernel/boot output (but only until the sceen came up). One of the lines output is:
COMMANDLINE= console=ttyS0 root=/dev/mtdblock2 mtdparts=sharpsl-nand:7168k@0k(smf),5120k@7168k(root),-(home) jffs2_orphaned_inodes=delete EQUIPMENT=4 LOGOLANG=1 DEFYEAR=2005 LOGO=1 LAUNCH=q
Anyone have any ideas?