Note that if you're trying to do this on a Snow Leopard Server machine (at least with 10.6.4), you'll find that there is no commented-out section in /System/Library/LaunchDaemons/com.apple.syslogd.plist (and that the plist file is stored in a binary format).
However, copying and pasting the key that Doug quotes above will do the trick, although first you will need to convert the format of the file to text thusly:
sudo plutil -convert xml1 /System/Library/LaunchDaemons/com.apple.syslogd.plist
...and you should probably convert it back afterwards (conversions happen in situ):
sudo plutil -convert binary1 /System/Library/LaunchDaemons/com.apple.syslogd.plist
...then reload the launchd daemon per Doug's instructions.
Afterwards the full plist file should read as follows:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>EnableTransactions</key>
<true/>
<key>HopefullyExitsLast</key>
<true/>
<key>Label</key>
<string>com.apple.syslogd</string>
<key>MachServices</key>
<dict>
<key>com.apple.system.logger</key>
<true/>
</dict>
<key>OnDemand</key>
<false/>
<key>ProgramArguments</key>
<array>
<string>/usr/sbin/syslogd</string>
</array>
<key>Sockets</key>
<dict>
<key>AppleSystemLogger</key>
<dict>
<key>SockPathMode</key>
<integer>438</integer>
<key>SockPathName</key>
<string>/var/run/asl_input</string>
</dict>
<key>BSDSystemLogger</key>
<dict>
<key>SockPathMode</key>
<integer>438</integer>
<key>SockPathName</key>
<string>/var/run/syslog</string>
<key>SockType</key>
<string>dgram</string>
</dict>
<key>NetworkListener</key>
<dict>
<key>SockServiceName</key>
<string>syslog</string>
<key>SockType</key>
<string>dgram</string>
</dict>
</dict>
</dict>
</plist>
One more note: if, like me, you want to send your AirPort base stations' (and/or Time Capsules') syslog outputs to your server, they use facility 0, which cannot be changed. This means that they will be automatically logged to /var/log/appfirewall.log because of the following default entry in /etc/syslog.conf:
local0.* /var/log/appfirewall.log
On the Server version of the OS, you can safely change the filename to e.g. AirPort.log once you've issued the following command:
sudo touch /var/log/AirPort.log
...since Apple's Application Firewall (socketfilterfw) is off by default (and should remain off on a server—ipfw is all you really want). I'm not sure if it's possible to reconfigure socketfilterfw to use a different syslog facility.