We support SCP to copy files securely.  For instance, on a Mac or Linux:

scp somefile root@ip.of.the.chumby:/mnt/usb/.

...will copy the file to a USB dongle on the chumby.  I believe several file transfer clients that support FTP also support SCP.

I started a thread when I was looking for a Mac OS-based SCP GUI for the Chumby:

http://forum.chumby.com/viewtopic.php?id=1616

I've been meaning to let everyone know that I finally found a fantastic solution. The program is called Interarchy and is available here:

http://nolobe.com/interarchy/

I worked directly with the developer to make the code patches to get it working with the chumby and it works great. Here's a screen grab of my chumby as seen thru the UI:

File and directory manipulation is drag and drop easy. You can open files directly in your favorite text editor and save directly back onto your chumby's flash drive or a writable directory. You can bookmark directories, etc. This program rocks and the developer was great to work with. Well worth the price.

If you want to try it and it's not working for you, email me and I'll send you the patch.

username: root
no password

Note that you'll have to launch SSHD on the chumby using the "pi" panel.

Interarchy 9.0.1 includes this patch:

http://nolobe.com/interarchy/

Connect via the SSH protocol and you should be able to transfer files to and from your Chumby.

Enjoy,
Matt