Oops, this post published a bit early, so if you saw an old, empty one, you should have refreshed ;)
I'm upgrading my computers from a year-and-half-old MacBook Pro (2.33 GHz, 3GB, 160 GB) to a current, updated one (2.5 GHz, 4GB, 200 GB). I thought I'd note down the slight differences between their hardware that I've found.
Many keys are now labeled more explicitly on the keyboard. "ctrl" is now "control", "alt-with-weird-symbol" is now "alt / option", and "apple-icon-with-cloverleaf" is now "cloverleaf / command". "left-arrow-meaning-backspace" is now "delete". "weird-looking arrow" is now "enter / return". "arrow-hitting-a-wall" is "tab". Good... I guess. Especially for new users. Then again, arrow keys don't have "pgup" / "pgdown" / "home" / "end" labels.
There is no more "return" (next to left "command"). Instead, it is now an alt/option key. Nice, because the first thing with the previous models that I did, was to get DoubleCommand and remap that key to be alt/option anyway, so there's one less action.
Alternative functions for function keys are in different places. While F1 and F2 are the same to change brightness and contrast, mute/volumeup/volumedown used to be F3 through F5, which are now F10 through F12. F3 and F4 appear to be Exposé and Dashboard. So now I have two functions for them: fn-F3 and fn-F4, and F9 and F12, two pairs that do the same thing. Oh well.
Another function key change: there is no more Num Lock. Which I never used anyway.
I like that there are now keys (fn-F7 through fn-F9) to go to previous/next song and pause iTunes. These are global shortcuts. I always had to install Quicksilver for that, but now I no longer have to, as I can use these keys, which is also faster, as Quicksilver iTunes control keys sometimes worked with a lag.
Not really a difference between the models, but the keyboard layouts. The old one had Swedish layout, the new one has US. But I use Estonian layout with both :) which is a bit tricky with the US one, as it has one less key. I have yet to find how do I do the "greater/less than" signs. The pipe character is shift-alt-tilde :) And around the Enter key, the layout is also different. The reason why this post got prematurely posted in the first place was that I hit Enter when I meant to hit apostrophe... have to retrain my finger memory a bit.