Howdy,
I purchased a 12" 867 PBook last June. I've been using it full time since then: I'm both a technology consultant (database development, unix administrator, open source implementation) and media developer (motion graphics, sound designer, video editing, etc.). I definitely put that little thing through its paces and loved every minute of it. Yes, the 12" screen got tiresome after a while, but I tend to have 3-9 applications running simultaneously depending on the project I'm working on. Exposé helps, but it's still cramped.
Three weeks ago I jumped on the 1.25 GHz 15" PBook, selling for $1,999 (at the time) refurbished.
This new PBook blows the 12" away in every single possible way, except for one: the size. The form factor of the 12" is perfect to pick up and move around. It's only 4.5 lbs. and battery life is really great. I feel like I've got to be more careful with the 15", than the 12".
However, I must reiterate that the 15" blows the other laptop completely out of the water. It's an absolute dream to work with, the screen is incredibly bright, crisp and clear, all applications run like sweet butter. I can't even believe how f'kin stark the improvement is.
So, my advice is this: if portability and battery life is of great importance, definitely go for the 12". You'll save some bucks, have use of a SuperDrive, and you'll have a ball carrying it e-v-e-r-y-w-h-e-r-e. However, if you really need to squeeze Power out of your Book, run don't walk to the 15". You can always pick up an external SuperDrive later for $200-300.
Final point, my original 12" was Rev. A. There were a couple of architectural changes made with the Rev. B 12" PBooks and forward, so perhaps the tremendous gains I experienced had something to do with these changes. As others suggested above, go to a local dealer if at all possible, and hang out for a while to actually use it.
Best of luck and enjoy whatever you choose!
Best,
Logicat