That would be cool: 3 sizes, all two inches apart and all widescreen.
However, that nice little 12" screen certainly allows for a tiny, compact laptop!
In terms of getting the 12" on a "level playing field" in relation to the 15" and 17", if I had to choose between a 13" widescreen OR the 12" having L3 cache and DVI, I'd TOTALLY go with the latter.
For me, it would be 50x cooler to have something with every bit the power, capability and pizzaz as the two larger PowerBooks in such a small, convenient package.
No compromises. Again: ideally, the ONLY thing that should separate the three PowerBooks is screen size/resolution. And maybe something like base RAM or hard drive capacity (which could, of course, be tweaked via BTO). But the same core set of goodies should exist across all three of something in the same product line, IMO.
Then it's pretty much down to screen size: which one would you prefer (since you're not taking any sort of other hit in regards to performance or I/O, you can simply buy the size/weight/screen you'd like to tote around and use).
THAT should be how a PowerBook is purchased. Perhaps it'll get to that, maybe sooner than later?
I'd not even CONSIDER the 15" or 17" if I knew that the 12" I was buying was as full-featured and smartly-equipped as the two larger ones. Talk about a no-brainer!
Then I'd just buy a 20" Cinema Display for home use!

How incredible would THAT be: a 1-1.2GHz 12" PowerBook with 1MB of L3 cache, 1GB RAM, 60-80GB hard drive, 2-4x SuperDrive, 32-64MB graphics, FireWire 800, lighted keyboard, etc. hooked - while at home - to a 20" Cinema Display! When you're ready to go, disconnect the speaker, display and any other cables (or buy a dock), close it up, drop it in a little sleeve and hit the road.

Talk about "best of all worlds".