I found this on the MacNN website and it still works (Mac OS X 10.4.11):

1) Create a text file with TextEdit on your dekstop and call this file “ftpchroot” (without the quotes)
2) Put the names of the accounts you want to be restricted into the file, 1 per line
3) Press Shift-Command-K and type “/etc” to go to the “/etc” directory
4) Copy the file “ftpchroot” to the “/etc” directory (you may need to enter your password)
5) Turn off FTP sharing in the Sharing control panel of the System Preferences window and turn it back on

Any user whose account is listed in this file will be limited to his/her dirrectory when they log on. They can only move “down” in sub-directories (if they exist), but they can’t move “up” any higher than their root dir. This works on OS X client versions only, NOT on OS X Server.