There is a very good article on the issue located at AFP548
In a nutshell, in OS X.1 you used the inetd.conf file to enable TFTP, in OS X.2 you now modify the files inside the xinetd.d directory. Also, to upload a file to an FTP server, a file with that name has to already exits. I guess this is a security thing to prevent someone from filling up a TFTP server
I got mine working with this article no problem.