As far as I remembered it was aluminium... I'm not saying I can't be wrong...
Haha, I thought you were joking.
The 12" and 17" PowerBooks were the first aluminium Mac products, as far as I'm aware. For whatever reason, Apple didn't introduce a 15" Al PowerBook for another 8 months or so, and this is after 15" PowerBook users had waited almost a year for a proper update. Well, they got a speed bump to an overclocked G4 processor, while the 12" and 17" got an upgrade to aluminium.
The 15" stayed with Titanium, which was only a problem because the paint flaked off....