Hey I was wondering the same thing...
I can CONFIRM, 100% for sure, that what Altemose said above is true - the connector (cable) on the 14" is about 1/2" longer than the 12" - the 12" keyboard will NOT work on the 14".
That said, here is some more of the story... I received the 12" Friday - it had been advertised as "for parts," but listed as "powers on and functions." Well, the machine I got did neither. Out of the box it had no power at all. I decided to test the keyboard on the 14" (it works, btw, but won't physically fit) and I noticed that the trackpad connector wasn't connected; that got me thinking that if THAT was disconnected, what else might be? Seeing that having to ship it back to get a $20 refund (it'd cost me $10+ to ship it back) just wasn't worth it, I decided to rip it apart and see what else was going on...
Turns out the DC-in board, LCD, and, YEP! - the power button - we're all disconnected. Also, there was no hard drive in it (no loss though; I had a bigger, faster drive I was going to put in anyway). After making these "repairs," it fired right up! So now I have a perfectly working 12" iBook (minus the Airport card - I'm using a USB stick right now, with an AP card on the way).
The specs:
The 14" is the last one - the 1.42GHz model. It has 1GB of RAM and the stock 60GB 4200rpm(!) hard drive. It's currently running Leopard, though I'm not using it due to the lack of a keyboard...
The 12" is the next to last one - the 1.2GHz model. It has 768MB of RAM and I installed a 60GB 5400rpm hard drive. It, too, is currently running Leopard.
EDIT: Oh, I forgot! Both of them, quite surprisingly, have very good batteries! I get over 3 hours on either of them and both have less than 150 cycles (144 for the 12, 120-something for the 14).
--QUESTION: Should I run Leopard on these or should I go for Tiger?--
I installed Leopard because that's the disc I found first lol - however, I still feel like it would be faster with Tiger. I've disabled the transparency, the 3D dock, the dock animations, and dashboard (but not spotlight) and its still feels a bit more sluggish than it should be. Thinking of trying Tiger tonight...