If you check the "Remote Login" box in the Sharing panel in System Preferences on a Mac OS X computer, you will enable the built in OpenSSH server. In previous versions of OS X, you could configure which port it listened on by editing the "Port" line in /etc/sshd_config. In Mac OS X 10.3, sshd is (strangely) run from xinetd, so the Port configuration line no longer has any effect.*

xinetd appears to rely on /etc/services to figure out port numbers, so the easiest way I found to change the ssh port is to change it there. You can view /etc/xinetd.d/ssh, and could possibly add a port line there, but the xinetd.conf man page says "If this attribute is specified for a service listed in /etc/services, it must be equal to the port number listed in that file.". So you may as well just change it in /etc/services I think.

To modify the file in place, you could use this command:

sudo perl -pi -e 's!^(ssh\s+) \d+/!$1 2200/!' /etc/services # changes it to port 2200

Then you need to HUP xinetd:

sudo kill -1 `cat /var/run/xinetd.pid`

*Actually, if you want to start sshd manually, ie not via the Sharing GUI, it will ofcourse run as a daemon and use it's configuration file as expected. But if you do it the way described above, you can still turn ssh on and off with the OS X GUI, and have it started automatically, etc.