Well, most of the firmware is actually contained in a closed-source binary blob (MainApp).*It's quite hard and close to impossible to work on it, and Samsung won't help - it contains all the DRM-related stuff and that's not something everyone should be able to edit.
So basically there's a basic distro running on it and supporting stuff such as displaying PNG, managing Wifi, or listening to music.*Everything else is inside this single MainApp blob, meaning there's no way to edit the interface, the music front-end, the fonts used in the eBooks section, or the Library management (ie.*ability to create subfolders). Even the display itself and the touch screen are totally handled by this MainApp.
I can't really do much more than what actually made it into Foxfirmware.*I'm not a developer, my knowledge of programming languages is pretty limited as well as my ability to create alternatives to what Samsung published.
Telnet will allow you to remotely connect to the E60 and run your own scripts in the Linux environment, using Bash.*It's then up to you.*You can browse around the filesystem, copy files, edit them providing they are not in the read-only fs, or even run stuff such as "mplayer [url]" to listen to online music if your device's wifi is enabled.
The guys over at hardware.fr (and http://code.google.com/p/e60-open/ ) are trying to completely remove MainApp and rebuild a whole custom system. This will take time, but allow for far greater customization.*Right now, the hard part is creating a driver to handle the e-ink screen correctly. But afterwards, maybe we could get something like
OpenInkpot running on the device.