Sounds like a great idea - many professionals are carrying computers, mobile phones, iPads, and the associated chargers. Synchronizing data and files between them is a nightmare and the overlapping capabilities are adding to the weighty burdens on our backs as we travel. A device hierarchy that enabled just the necessary tier to be utilized would be a great advance.
I bought a phone over a year ago that has a Netbook-sized dock accessory. You can literally turn the phone into an 11-inch Firefox-powered browser. Of course the dock is actually more expensive than just buying a netbook, so it's essentially useless.
The problem with hybrids is typically the price point. Sure, they are useful, but when you spend more on getting a device that can be two things than you would have spent on buying the two things, you've lost yourself in the woods.
At some point in the not to distant future, your phone will have the computing power to do pretty much any non-specialized task needed. When that is the case, why would we need anything other than a keyboard (or whatever entry device is appropriate for a given application) and a display.
There are a few issues to be ironed out, like security and data backup. No matter where you go, if you see a community screen / keyboard, say in a coffee shop, you could sit down and start working at exactly the spot you left off when you left home or the office. No pulling a tablet or laptop out of a bag. No digging your phone out of your pocket or purse - and no need to worry about keeping data synchronized.
Your data and computing power just stay in your pocket but are always accessible.
So this is an attempt to combine several of the devices I now use. If my desk top, lap top, tablet and phone (plus the PDA I sometimes still use) could all be one device, that would be convenient. I guess the thing I would worry about most would be that if it is one device and it breaks, then I lose all of them. My lap top today told me that the hard drive is about to die. The second hard drive in my desk top died a couple of weeks ago. Maybe it is better to move to the 'cloud' or use solid state drives. Anyway, I guess I am for it as long as I have a back up with a quick replacement of any thing that dies.
One problem with a tablet / phone hybrid is the desire to make the tablet side big enough to be useful without becoming unwieldy as a handset.
One solution would be to physically decouple the handset from the tablet, making a 3-tier system, stand-alone phone with basic functionality, phone with enhance functionality radio-coupled to the tablet and finally a fully docked system with keyboard, mouse, display and handset.