I've used both the Blackberry Curve and the iPhone for several months now, and here's my take:
The Curve is a more durable, more reliable, and ultimately a better device for professionals. It is very light, has outstanding battery life, good UI design, and I never have to wonder if it will work. The hardware, software, and blackberry network have evolved considerably over nearly 10 years and it shows.
The iPhone is more beautiful, utilizes more advanced technology, and simply does more than the Curve. However, typing on the blackberry's tactile keyboard is simply easier and more precise. I had an especially broken iPhone but even my new one hiccups fairly frequently when connecting to my home network. I feel like I should use a case for my iPhone so I don't scratch it but who wants to hide that beautiful design in a case?
Surfing on the iPhone is awesome. Surfing on the Curve is just functional. I own both devices at the moment but if I had to carry one device with me for business it would be the Curve. On the weekend, I want the iPhone.