I reformatted my discs and installed 10.4, after backing up all my goodies to an external FireWire disk. On rebooting, I could see the disc in disk utility but not mount it. It gave errors about key-length etc. Trawled the 'net looking for a solution, tried all the terminal tricks but to no avail.
However, I discovered by accident that I could mount the disk on my wife's eMac over our LAN -- probably because it mounted Read Only. I have copied all the goodies over from her eMac to my new disk, and now I am back in business. It was slow going, but it worked minus the files that mangled it in the first place.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050529013212690