Nope, does not work either, at least for me.
I'm new to the thread, but my problem is just the same: invalid PID all over the place!
After your post, I took the prc file and opened it on my PC. that worked fine, although the file was on a USB key and no files were created in My Ebooks folder. Nevertheless, something happened, as I now have a new 4kb .mpb file on my key! I then went back to my mac and tried to run mobidedrm, but to no avail... Snif!
EDIT Or rather yes it does!!! This time, I connected my pc to the net and downloaded the book on it, and that time it worked! Thanks a lot for that!