IMHO there will be one more G4 refresh of the PB line sometime in the next 2 months, probably with speeds around:
12" 1.25GHZ
15" 1.25GHZ
15" 1.33GHZ
17" 1.42GHZ
...Not huge boosts at all, but I would hope for better harddrives as standard (5400 RPM), a GPU update, the RAM in 1 spot, not in 2 by default like it currently is in the 15", and finally maybe a faster superdrive (though I don't even know if these are available.
...not overly optimistic, but I think we will see this for the final G4 powerbook, then the G5 powerbook at MWSF 2005 in January.