Convince on the 12 inch
Yeah, I'll try to make a quick go of convincing you.
I currently use a rev. A titanium, and quite frankly it is a great machine. Still has some power (though I have upgraded the ram and harddrive) and is only one inch thick.
However, when you get a portable you want it portable, and carrying this thing around all day with a thin shoulder strap on a briefcase can get tiring and it doesn't always sit well on planes and other cramped spaces.
Well, what about the 15 inch monitor? When I am web surfing, emailing, using iTunes, iPhoto or writing, I don't use the 15 inch. When I am using photoshop, dreamweaver, iDVD, etc. I need more than a 15 inch. By my use, the 15 inch is a bad compromise.
Being an educator I can get a superdrive 12 inch for 1599. I then can get a 17 inch Samsung LCD with DVI connector for around $500. Now, I have a monitor solution for all the work I do. Granted if I had to do large amounts of photoshop or dreamweaver on the road, this wouldn't be good, so I am not trying to get into a debate about what machine is better. I'm just saying if you look critically at what you do, sometimes the 15 inch is a mistake.
There are plenty of scenarios where the 17 or 15 inch is better laptop for a certain person, I'm just giving an example of usage the 12 inch fits nicely.