Hmm, looks like I've run across a fictionwise mobipocket book that doesn't successfully decrypt (though the script runs/works fine).
I've successfully utilized mobidedrm many times -- and reran the scripts against old books just to make sure I hadn't dorked something up. I also brought out fresh versions of the scripts just to make absolutely sure.
Inspecting the file with a hex editor, everything looks good until a bit after the <head> element, with parts of text that are fine and parts that are garbled. Looks like either something odd with the copy I bought from fictionwise, or there's a weird variant that the script isn't picking up.
Since I'm on OSX, I'm sorta hosed for reading my book -- anyone out there interested in taking a gander at before/after versions of the book? This might be something that a certain someone might want to take a look at for additions to the script.