I was having this common problem.  After doing some research and almost every thread saying that it was a hardware compatibility issue, I started playing around.

I booted up with the PTVUpgrade companion cd (the LBA48 supported mfstools disc available for free) because I knew that it detected my hardware and worked perfectly.  I ran PTVbake-special and it seems to be working A-OK at this point (it's still at "Cleaning up restore.  Please wait a moment.")

So, that worked for me.  This is with the HDR-112 InstantCake disc.

My question for the developers is, why not use the same drivers from the PTVUpgrade companion CD on the InstantCake CDs?  The drivers used in the PTVUpgrade disc seem to be much more versatile.

It also might be worth mentioning to customers that the InstantCake disc will not work with USB keyboards, or at least it didn't with mine .  I suppose this might go back to the hardware support issue, though...