A PowerBook 3400:
Now a PowerBook 5300:
And the 1400:
Now the 603e-powered Duo 2300c:
Note the slight differences between the 3400 and 5300 - they are very slight. The lower case is essentially the same, except the 5300 is just stubbier and it has a smaller screen. I have seen both in person, and the 3400 is definitely larger, but you can tell that they're based on the same design. One thing that adds to the 3400's bulk is the in-screen speakers. There are speakers in the back on the screen that make it bulge out.
Also, the 68040-powered PowerBook 190 and the 603e-powered PowerBook 5300 used the same case. Then you have the 3400c and the Kanga, which used a slight modification of the 190/5300 case. One could say that the same design carried over all the way from a 68040 machine to a G3 machine.
Note that the 1400's case is different from any of the others. It was nicer, more compact, and it had a really good keyboard (I used to own one). Despite having an 11.3" screen vs. a 12", it seems significantly smaller and sharper.
The entire Duo series used the exact same case, all the way from the Duo 210 (25 MHz 68030, grayscale) through the 2300c (100 MHz 603e, active matrix color).
The "new" Apple has also kept case designs the same even when upgrading the processor, at least when they went from the blue G3 to the PowerMac G4. The color changed but it was the same case.
Overall, I think Apple just changes the case every few years, and they change the processor every few years, and it's usually a good idea to change them at the same time for added effect. Since Jobs came along, it's much more that way - he's been really increasing Apple's publicity. I mean, look at this... people are arguing that the Kanga never even existed, and it's most likely because it was such a quiet upgrade (despite involving a whole new processor). But everyone remembers the Wallstreet.
Compare to the Pismo to Titanium upgrade. Almost nothing changed, other than the screen, case, and processor. The hard drives remained the same, base RAM remained the same, processor speed stayed... but it was still a big update because of the new case AND new processor at the same time.
EDIT: IMHO, there won't be a 970 PowerBook before a 970 PowerMac. It's just wishful thinking.