When the Ultra Mobile PC (UMPC) was launched in the middle of 2006, it didn't exactly meet with an enthusiastic reception.

Whilst the idea of a paperback book-sized PC you could use with a pen like a notepad sounded promising, the first generation of UMPCs just didn't hit the mark. They were too bulky, with too short a battery life. And they were too expensive. Yes, they were fun to try (as a lot of new technology ideas are) but not particularly useful to own.

However, just because the first UMPCs didn't quite get portable computing right didn't mean the idea of a full Windows PC you could carry around you like a Filofax was a bad one.

But do you really need Windows in your bag? This is the question asked by the latest breed of all-singing mobile phones, in particular Apple's iPhone. The iPhone claims to be almost a full computer, too. It runs Mac OS X, although on an Xscale PDA processor rather than a notebook CPU.

So if the iPhone is essentially a little portable Mac, and the UMPC is a portable PC, why has the iPhone succeeded where the UMPC failed? Both can play music and video, and you can get UMPCs with built-in GPS, such as Asus's R50A, which can give you a map showing exactly where you are.
Let's forget the size difference for now - although this is clearly a major factor. The iPhone may be pocket friendly, where the UMPC would require trousers baggier than MC Hammer's to accommodate it.

The obvious difference is that Apple realises that the battle to be the people's everyday computing device was won some years ago. And it was won by the mobile phone.

Sure, Microsoft has Windows Mobile, and that's not doing too badly. You can buy quite a few decent phones running it nowadays, although pure PDAs appear to be dying out, with Dell killing its Axim range in April. But Windows Mobile is not really Windows, anyway, it's Windows CE, which is an entirely different codebase and can't run the same applications.

In contrast, the iPhone runs MacOS X, right? Well, no, not really. You definitely can't run desktop MacOS X apps on it. In fact, you can't officially run third-party apps on the iPhone at all, just unofficial hacks, although in October Apple did announce it would be releasing the SDK for this in February 2008.

So the iPhone's success is not about the extra software you can run on it, which is the primary selling point for the UMPC.
Despite the fact that the iPhone is basically a large PDA phone, it focuses on fun and style, not processing power and extensibility. After initial reports to the contrary, the iPhone has arrived with Exchange email support and Office compatibility. But it's really the funky iPod features and built-in Google Maps which make the iPhone more than just a phone.

The UMPC, on the other hand, is a device designed by people who only begrudgingly know what leisure is. It's a laptop without a keyboard; another incarnation of the Tablet PC concept. It has some fun abilities, but its big selling point is that it can run Windows apps, in particular MS Office. In other words, you will have even fewer excuses for not working 24 hours a day.

So who needs a UMPC any more? Who needs, or indeed wants, to carry a complete computer around with them at all times?

Well, some of us do. But most of us don't. Most of us just want an all-in-one mobile entertainment device you can actually fit in a pocket. And that's why so many people have gotten all hot and bothered about the iPhone, even queued up for hours to buy one, where the UMPC has come and gone with very little fanfare.