MacBidouille claims to have reliable information on upcoming Powerbook updates.
They claim that the long-awaited Powerbooks will make an appearance at the "end of the next month" and will be produced with the PowerPC 7457 from Motorola. Speeds are expected to be 1.1, 1.25 and 1.33 GHz with USB 2.0 and Firewire 800 throughout the entire line.
This information is consistent with expectations from The Register that this processor will be used in upcoming Powerbooks. Availability of the 7457 is expected Q3 2003, per previous reports with a top speed of 1.3GHz.
A recent Page 2 Rumor, however, pegs the Powerbook releases in September (presumably at Apple-Expo in Paris).