Last night when I messed up my account, I was trying to (re)set-up SVN on my laptop. I was following these instructions. They're pretty good, but they leave out how to actually get the launchd daemon loaded and working. I found these instructions on creating launchd daemons helpful as well as these on getting started with launchd. Here's specifically what I did after I'd created the svn.plist file:

• cd to /Library/LaunchDaemons
• Start up launchctl as root

sudo launchctl

• Load the svn.plist file

launchd% load svn.plist

• You can list the loaded jobs now to be sure it's there using the list command.
• Start the job:

launchd% start svn

Now, the daemon is started and should load each time the machine reboots. Since it's "on demand" it will actually start up the SVN server each time it's needed rather than having it always running.