Instead of using a GUI application, such as ZTerm, to connect to the serial interface, you can use the command line tool minicom. This has the big advantage that it can be used when connected to the computer via SSH.
You can download minicom via Fink, or compile it from the source. I used minicom-2.1, which can be found on here to compile it on Mac OS X 10.3. In the Terminal, after downloading and decompressing, do:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20041026113304907