I just had two, back to back, problems on Mac OS X.

First my printers disappeared, and i wasn't able to add new ones; several minutes after clicking the "add" button i'd get a error message about "server-error-service-unavailable". Googling found that this was a cups error message. It turns out there was cruft in /var/spool/cups which--after deleting (i didn't delete the tmp subdirectory) and restarting cups--everything took off working fine.

The other problem was that spotlight searches were returning nothing. I finally found out about mdutil and tried running both of:

mdutil -s /
mdutil -i /

to check the indexing status and try turning it on. The status returned empty, and turning indexing on gave the error: Could not set indexing status for volume. I took the advice of removing /Library/Spotlight and /.Spotlight-V100 and then running "Repair Disk Permissions" in Disk Utility. Restarting Metadata saw spotlight working again.

In both cases, the instructions i were following suggested doing the above and then rebooting the computer. Reboots are evil. The desired effect of rebooting in both cases was to get the service processes started again. It turns out you can do this quite easily manually. OS X has an equivalent to the /etc/rc.d/init.d scripts found on most decent unix systems, but a little harder to find given they are tucked away in /System/Library/StartupItems. To get the above two services started again, i just had to do:

cd /System/Library/StartupItems/PrintingServices
./PrintingServices start
cd /System/Library/StartupItems/Metadata
./Metadata start

Reboots sold separately, batteries not included, not available where prohibited.