If you have any interest at all in the "market" for smart devices, you should be reading Mobile Opportuntiy by Mike Mace. A lot of the talk I've participated in with regard to the iPhone has been where exactly in the market it will fall and which devices will it therefore slaughter. Mace's analysis of the smartphone market here is spot on.
If you're not a smartphone industry wonk (or wannabe wonk), then just skip to the bottom, where you'll find the surprising "Zone of Death". Best. Demographic Target Descriptor. Ever.
I believe the market for mobile data devices (smartphones, PDAs, mobile game machines, iPods, etc) is not structured the way most people think it is. A lot of new mobile products fail because they're not designed for the real market, or because they target imagined customers who don't really exist in large numbers.