Although, if you include upgrades, you can reach 1.92 GHz with a (now discontinued) Daystar upgrade on any of the Aluminum PowerBooks.
It's odd, they have discontinued the Aluminum and Titanium upgrades, but they still make upgrades for the Pismo and Lombard! I'd rather upgrade my 12" PowerBook to 1.92 GHz than upgrade my Lombard to 550 MHz...