I had the same problem when I purchased my Macbook back in December.
My 10.4 running G4 Powerbook wouldn't show up on any 10.5 running Intel Mac. It would show up on a 10.5 running PowerPC computer though. I never could get around it so just transferred files a different way. I put my new computer into target disk mode and plugged in to the Powerbook.
Best of luck to you, hopefully someone can come up with an answer for you.