Don't listen to those 15 inchers with their l33t speak, it's all jargon

J/k, just had to find a way to use l33t for once...anyway, onto what you may care about.
[1] What is the real-world battery life of the 12" PowerBook?
- Using it day in and day out here in France it can easily watch nearly two full dvds (assuming 2.2hr per) while spinning the discs, more so if the vid is on your HDD. Usually I can 4+ with low brightness with airport on and browsing the net.
[2] What is the real-world battery life of the 15" PowerBook?
- Don't know, don't have one.
[3] Will any LCD monitor work with the PowerBook (as long as I have the necessary adatper)?
- Yes, given the proper adapter (no need for a dvi external from 15in)
[4] Do all PowerBooks sold at the Apple store come with Tiger preloaded?
- Any PowerBook you currently can buy in an Apple store will have Tiger on it, unless there is some odd old-school blowout that I am unaware of.
[5] Is the AppleCare extended warranty a "must"? It adds a considerable cost to an already expensive machine. (I've never had to have any previous notebook repaired)...
- You can purchase AppleCare 364 days after the purchase of your PowerBook if you so choose, just has to be within the first year to extend the coverage to 3 years total (including the first...NOT four). I did not purchase AppleCare for my 12" however I still have a few months to decide and depending what is on the market at that time, I may be selling my 12"PB and updating by then, so I will wait and see, but zero problems here, thing works like a charm.
I'll save you more rambling about 12 vs. 15. It comes down to a preference of what you want/need and generally what you think you may prefer. I took what I thought was a gamble at the time and went 12". Today, having used it for months, even if it cost more than the 15, I would still buy a 12 today. Hope it all helps, either way you are getting an awesome machine, be confident in that.