Has anyone flashed an Android build (i.e. a Crespo build) onto a Nexus S phone manually before? How does one do this?
It's possible to inject your system.img into a previous nandroid backup done from clockworkmod and then restore that backup with the new system.img.

Prerequisites:


Installed clockworkmod recovery
desired system.img saved onto sdcard
do a fresh nandroid backup


To install the new system.img:


boot to recovery
create a nandroid backup to let's say /sdcard/clockworkmod/backup/[currenttimestamp]/
replace /sdcard/clockworkmod/backup/[currenttimestamp]/system.img with your new one
compute the md5sum of your new system.img
update the md5sum in the system.img line in /sdcard/clockworkmod/backup/[currenttimestamp]/nandroid.md5 file with the value from [4.]
restore your altered backup
if needed, do a wipe