A well written article by Duncan. And pretty much exactly what I've been saying for a while about the iPhone and it's potential rivals.
Been saying since iPhone-1, if they include a slide-out keyboard on the iPhone, you'll have a very, very good package. But the lack of a keyboard still is the greatest limitation of this terminal.
I also suspect the biggest threat to the iPhone is going to be the Bold. Should have mine soon but the short times I played with it, it really impressed. The G1 is also something to watch, pity it's an ugly duckling.
One aspect of the iPhone that caught me (pleasantly) by surprise is the number, price and quality of the apps for it on the iTunes Store. I find myself buying and downloading a number of really cool little applets and at 99c, you don't really think twice about buying them.
OS's have always stood or fallen on application availability (Windows being the ultimate example) plus the ease of getting hold of them and here (as a non-Mac / iPod user) the collection of apps on iTunes came as a really nice bonus.