I recently had to re-partition a 10.5 server just after the install had been done (the boot partition turned out too small). The server was completely headless at that point and I couldn't just resize the parts using diskutil because this was all on RAID 5.
There's an interesting “feature” in Server Assistant - the server will eject any optical media until you get to the serial number part. I tested this several times and the results were consistant. Anyways, you have to get to the serial number part, then insert the install DVD, then set that as the boot partition:
bless --folder /Volumes/Install Mac OS X Server --setBoot && reboot
After which you can just ARD to the box and re-partition and - install. This confused the heck out of me for a while - posting it here as a “warning”. :-D