I am not sure if this was posted before, but you can now download the GPL'ed sources of the PRS-505 firmware.
Link:
http://www.sony.net/Products/Linux/D...tegory3.html#2 (you have to accept the disclaimer to get the right cookies, then click on "Portable Reader System").
Files that seem newer/have been added:
8track20070714.tgz
EBOOK_1_2_0_P4.2_20070426_Linux_src.tgz
USBTG_EBOOK_20070727.tgz
src_libroi2s_070426.tgz
src_libroi2s_060713.tgz