Repairing Permissions

From Mac OS X Server FAQ

Some administrators repair permissions as a matter of course before and after any system updates, while others do not with seemingly no ill effects. In the event that a file's permissions become incorrect, according to the package that installed the file, repairing permissions would fix the problem. Things that can change file permissions for the worse include rogue applications, rogue administrators, and even rogue installer packages. If you find yourself confonted by any or all of these, then repairing permissions on a regular basis may not be a bad idea.

If you ever experience any general issues with your server, its also a good place to start... after checking your logs, of course!

If you boot your server and instead of seeing the gray screen with the Apple you see a prohibitory symbol (Ghostbusters circle, minus the ghost) permissions are screwed up on the drive. Restoring from Retrospect backups doesn't seem to keep the correct permissions. Boot off your Server Install CD. Open Disk Utility from there and repair permissions. Then reboot and you should get your Apple, and bootable system back.