Nigel Whitfield recently wrote a detailed guide on the hacking of the Sony Reader PRS-505.
The universal flasher
The PRS-505 employs the MontaVista Linux, making the customization possible and actually rather easy. With the
universal flasher, the firmware of the system can be updated to offer more customizations, and even running homebrew applications on the Sony Reader. However, due to the limitation of the unit’s processing capability and the response of the E-ink display, in the meantime modifications are mostly made to improve the way of reading.