You can use Apple Remote Desktop to connect to a machine that is booted from the O X 10.5 server installation disk by typing the first eight characters of the computer's serial number into the password field while leaving the user name field blank.
[robg adds: I haven't tested this one, and I don't know if it works with the Client installation disk as well.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080821173049121