TFTP Server on 10.6.6
Do you need to run it with any special settings?  By default, it runs chrooted in /private/tftpboot; if you want anything different in the setup, edit /System/Library/LaunchDaemons/tftp.plist as appropriate.  Then, activate it with:

sudo launchctl load -w /System/Library/LaunchDaemons/tftp.plist