Appleinsider claims that dual G4 PowerBooks may "see the light of day" if the G5 is unable to be incorporated into the current PowerBook chassis in a reasonable timeframe.
According to the rumor site, Apple is working with Cooligy and is shooting for a May 2004 G5 PowerBook release.
PowerPage.org had made similar claims in September... that they expect Dual G4 PowerBooks to arrive before the G5 makes its way into the PowerBook.