% ftp 192.168.10.10 Connected to 192.168.10.10. 220 192.168.10.10 FTP server (lukemftpd 1.1) ready. Name (192.168.10.10:robg): robg 331 Password required for robg. Password: 550 Can't change root. ftp: Login failed.A little Google searching found this page, which explains the cause of the problem:
Internally, the bug is caused by changing the effective userid of the ftpd process to the user logging in before invoking the chroot command. Unfortunately, the chroot command can only be done by the root user, which is the user into which ftpd is first launched. It would be interesting to see the details of the original bug to see if it was creating a security issue or just an inconvenience.The solution, also detailed on the same page, is to replace lukemftpd, Apple's chosen FTP server program, with an earlier bug-free version. Instructions are provided to build from source (you'll need an Apple Open Source ID to get the files), or you can use the author's precompiled binary.
Others will comment, of course, that you can avoid all these problems in the first place by just switching to proftpd, which about which I've heard good things (though I don't do much with FTP, so I haven't installed it myself). If one were to do this, however, I don't know of a method of changing the Sharing preferences panel such that it launches proftpd instead of Apple's built-in FTP server.

