Intel perhaps most of all, but a lot of technology vendors are pushing the idea of MIDs (Mobile Internet Devices) and Netbooks (essentially scale-down, low-cost notebooks). Intel's interest here is pretty straightforward: the more a mobile device resembles a traditional PC, the more Intel's x86 franchise gives it a leg-up. By contrast, smartphones are based on any number of low-power processors, typically something other than x86 architecture.
I'm skeptical that these categories between the smartphone and the notebook will amount to a whole lot.
The issue I see with MIDs and Netbooks in the general case, however, is essentially a matter of form factor.
One the one hand, smartphones fit easily in most pockets. The downside is a small screen and text input that is largely by thumb, rather than by finger. Furthermore, because smartphones have historically been built using such a hodgepodge of hardware and software--including browsers--Website compatibility has been spotty at best, even leaving aside the (significant) issues that a smaller screen area introduces.
At the other end of the scale are familiar notebooks. Even the more portable varieties have more-or-less full-size keyboards and screen. Besides relatively high cost and the need to maintain and update a full-fledged operating system on a PC, notebooks weigh a few pounds and pit in a backpack or briefcase form factor--not a pocket, however oversized.
Against this backdrop, one can imagine Netbooks that sit in a kitchen to look up recipes or a MID that functions as a mobile browser and entertainment gadget somewhat in the vein of an iPod Touch. However, these scenarios feel like stretching to me. The cellphone is ubiquitous and highly portable (and smartphone browsers will get better). The notebook is well-suited to keyboard input and rich Website display (and will inevitably get ever smaller and lighter).
What do the alternatives offer?
A MID is a form factor that is neither as portable as a smartphone nor as full-functioned as a notebook. A Netbook is a notebook that is underpowered and otherwise compromised. At a low enough price point, perhaps. But the One Laptop Per Child experience suggests that the most aggressive price points may well be too aggressive to be practical.
In short, at least in a market where almost everyone has a cellphone and notebooks are the full-function PC of choice, it's hard to see the compromises of the MID and the Netbook as anything but too much pain for too little gain.
All that said, I'm now going to do something that used to intensely annoy a former editor of mine who never let the facts interfere with a good argument. I'm going to qualify my skepticism. By analogy, people ride and pedal all manner of vehicles. Some, such as bicycles and cars, are clearly mainstream. A few are true oddballs (unicycles). Some have very specific use cases (two-seater cars). Others are generally uncommon in the US but are relatively common in other locations (scooters).
Perhaps MIDs or Netbooks will emerge as the two-seaters or even the scooters of the computer world. Truly mainstream device? Probably not. But the uber-portable and inexpensive notebook, in particular, could find takers in the developing world or as a third- or fourth household PC in more developed nations. Especially as Moore's Law and other technical advances bring faster processors and bigger storage to even the most entry of price points.