The inch is a little different because the square nature of the screen makes it so that the notebook is significantly smaller.
Just a note on the difference between a 13.3" MBP and the 12" PB.
The Powerbook's screen is very different because of the aspect ratio alone.
For writing, I found the PB screen contained more of the length of a screen page than the 13.3 MacBook Uni.
Add to that the considerably larger amount of sheer mass of the 13.3" lappies than the 12", the 12" Powerbook as well as iBooks are still a better computer to tote around.
Anyone that needs the width of the 13.3 screen for film editing or watching movies is kidding themselves unless it is a secondary computer.
Apple will probably never return to the 12" format unless it is the size of their upcoming tablet. The PB is about 10 plus inches by 8 plus inches, this is very small -- and I am just recalling the specs from memory while im looking at my new used fourth PB that I bought to replace my unibody.
As for the iBook 1.33 12" vs the Powerbook 12" 1.5, the only differences are that the iBook had 256mb more built in ram but only a 32mb Video card, while the PB had the better 64mb Vram and less built in ram on the board.
That said, I am able to run my 30" ACD vis a vis my 12" Powerbook at a reduced resolution, but pleased that it runs it at all when I need it.