Asus held an event in London to show off the devices it revealed at Computex in Taipei recently, though sadly there was no Windows RT device to be seen.

Among the Zenbook Ultrabooks and Transformer Primes there was something innovative though, which was a near-final sample of the PadFone, which combines smartphone, tablet and Android laptop into one package.

The thinking is simple: why have an expensive smartphone as well as an expensive tablet, each perhaps with its own SIM card and contract, when the smartphone can power both? In the PadFone, the phone docks into the tablet, and the tablet clips into a keyboard case. As a final flourish, there is an optional headset stylus, a stylus with a Bluetooth headset built-in so you can answer the phone easily when it is docked.

Here are the three main pieces:

The tablet, note, is useless until you dock the phone. You do this by opening a flap on the back and dropping it in.

The tablet then works just like any other Android tablet, though it is heavier than average, and has a bulbous section on the underside.

Attach to the keyboard case, and you have a laptop.

The tablet has a 10.1”, 1280 x 800 screen with Gorilla Glass, a speaker and headphone jack, and a front-facing camera.

The phone has 1GB RAM, 16GB flash storage plus Micro-SD support, Qualcomm 8260A Snapdragon S4 Dual-core processor with Andreno 225 GPU, rear camera and its own front-facing camera, and runs Android ICS.

The keyboard adds USB ports and a card reader.

Each device has its own battery so a full setup has three batteries, or  four if you count one in the stylus headset. However you can have scenarios where the tablet is out of power but the phone is not, for example, which would be annoying.

I spent some time with the PadFone, scribbling on the excellent note-taking app which comes with it, and assembling and disassembling the unit to get a feel for how it works. There is plenty to like. The phone itself looks great and seems fast and capable. Docking and removing it is straightforward, particularly since the flap acts as a lever to eject the phone gently. Asus assured me that it has been tested for thousands of insertions. The tablet worked well too, though it is heavier than most and the protrusion which holds the smartphone is inelegant.

A winner then? I am not sure. It is interesting and innovative, but the mechanics need some refinement. Most people have a case to protect their smartphones, but for the PadFone you will either need to remove the phone from its case when you dock it, or else treat the tablet as the case, in which case it will not slip so easily into a jacket pocket or handbag.

The stylus headset is not just a gimmick; you will need this, or another Bluetooth headset, to make sense of using the phone when it is docked.

Some variations on this theme occur to me. After another generation of miniaturisation, perhaps you could design a phone so slim that it fits into the case more like an old PCMCIA card used to slot into a laptop, without an ugly protruding flap? Another idea would be to make all the communication between phone and tablet wireless, building just enough smarts into the tablet that it works as a kind of remote desktop into your phone.

The Asus folk present told me that the PadFone is first-generation and we can expect the concept to evolve. Another goal is to make a splash in the smartphone market, using the PadFone as differentiation from all the other Android devices out there.

Apparently the PadFone will normally be sold on contract, and while it will be bundled with the tablet, whose name is the PadFone Station, the keyboard and stylus headset will be optional extras.