Wouldn't it be interesting if a Sonnet Company made a processor upgrade for late PB G4's ?

That's not gonna happen, as all of Apple's laptops have had their CPUs soldered to the logic boards. Even if a certain PowerBook model could handle a faster CPU, it wouldn't be cost-effective from a labor standpoint to upgrade it, especially considering how tricky working with ball-grid arrays (http://en.wikipedia.org/wiki/Ball_grid_array) can be outside of a specially-equipped factory.

... all of Apple's laptops have had their CPUs soldered to the logic boards.

Very nearly true. Exceptions: Wallstreet, Mainstreet, Lombard, Pismo. There may be others, but none come immediately to mind.

It can be stated with certainty, however, that all of Apple's G4 and Intel laptops (as well as the entire iBook line, 680x0 PowerBooks, and most, if not all, of the early PPC PowerBooks) have had their CPUs soldered to the MLB and are not going to be easily or economically upgraded.

That's not gonna happen, as all of Apple's laptops have had their CPUs soldered to the logic boards. Even if a certain PowerBook model could handle a faster CPU, it wouldn't be cost-effective from a labor standpoint to upgrade it, especially considering how tricky working with ball-grid arrays
(http://en.wikipedia.org/wiki/Ball_grid_array) can be outside of a specially-equipped factory.

Daystar used to offer "CPU Upgrades" for a few G4 Powerbook models that involved desoldering the stock CPU and subbing the fastest pinout-compatible thing available out of the Freescale catalog. (And then overclocking it.) Here's a review of one of their fastest offerings.

They've all been discontinued, for the the obvious reasons. (The two biggest being they were labor intensive and pretty risky, and $500 to turn an obsolete and too slow machine into one that's still obsolete and only slightly less too slow and prone to constantly overheating wasn't exactly stellar value for the dollar.)

--Peace