Cell phones are technolgoy that we take for granted. Yet 15 years ago they were still very expensive and rare. Once the prices came down on both the hardware and the service, the super-fast adoption rate is a key indicator of the chell phone's "just right" technology. In case you remember what cell phones used to look like, here's a nice retrospective image set from msn.com. It was only about 10 - 12 years ago that phones got to the order of small that we know them today and that was still bigger than what we'd consider average today.
I would go a step farther to say that cell phones really turned out to be an enabling technology. The wide-ranging wireless network that cell phones are based on has grown and expanded to support an increasing volume of data transaction as well. This trend will clearly continue and cell phone like personal devices -- will be everywhere had by everyone.
They clearly still have a ways to go with the interface but they are getting better. I think the Apple iPhone is the start of the next generation of personal devices. I do have to say however that I like the current form factor of the small flip-phone and don't like the form-factor of the iPhone for being a phone. As a UMPC, the form factor is interesting and the multi-touch display is definitely trend setting.