I just successfully tested a fix for the fast-forwarding bug that was suggested by a Linux user (azara) over at VideoHelp.com:
Burn your disc using "ISO 9660 only". Do not use the "ISO9660 + Joliet" option, or the rewinding and fast-forwarding won't work.
This, ofcourse, does not address the slowness of rewinding/fast-forwarding, which is limited to 8x :(
For my test I used Nero 6.6 and used the following options on the "ISO" tab:
Data mode: Mode 1
File system: ISO 9660 only
File name length (ISO): Max. of 11 = 8 + 3 chars (Level 1)
Character set (ISO): ISO 9660 (standard ISO CD-ROM)
I unmarked all four "relax restrictions" checkboxes.
It works great and resolves the only complaint I had with the DVP 642 other than not supporting DTS sound.