Ya, I do this all the time. From my brother's house, he has 2 modded xbox's and we can transfer files back and forth through FTP by forwarding port 21 to our xbox's ip on our routers.
And forwarding port 21 to your xbox ip on your router can also make it accessible via ftp from anywhere that has an internet connection. So it can be used to host files for anything on the internet, basically run as a server, if you setup another filezilla username with read only privellages to only certain folders.
Also... forwarding port 8080 to your xbox's ip on your router can enable it to run as an http server. This in conjunction with editing and compliling your own cvs builds to change the webserver root directory to F:, can be very usefull for running websites, hosting all your online pics, or hosting or mirroring any file that you wish, and having it either available or not to the public.
This is extremely usefull for everything listed above and more, especially if you have a separate xbox with a decent sized hard drive that is setup for extreme cooling to run 24/7 as a server.
-Danny