At this point, I don't think it's unreasonable to expect that cell phones and tablets will ultimately merge into a single device, and that it will take over the role of the Roku and DVR as entertainment providers adapt to the online single-show/single-series subscriber models with devices like Chromecast bridging the gap between tiny screens and larger entertainment center experiences. More frightening is the idea that these will become the new house keys and primary home provisioning devices. Right now, people have coined the term "phablet" to describe this bizarre hybrid device, but I don't think that term is broad enough to describe what this will actually become.
Tablet manufacturers are capitalizing on the desire for larger screens on cellphones and better portability for tablets by producing e-Reader sized screens (here's where the "phablet" comes in). But, as these no longer fit easily into a pocket, many are opting for larger screens and storing these devices in briefcases, backpacks, or other types of carrying bags. But, as it is still inconvenient to produce such a device regularly to manage things like new meeting requests, text messages, emails, or other updates, the combination of the smaller device and the larger device retains popularity.
One attempt at an answer is the smart watch, a device designed to enable people to store away the larger device and remote manage it for the more frequent-access and graphically non-intensive tasks, such as answering text messages, accepting or scheduling meetings, and even managing twitter and email with simple voice commands. This removes the need for people to wear uncomfortable earpieces, and solves through external programming and software the lack of voice recognition via headset in most tablet technology. But, while tablets have gotten thinner, and lighter, and ultimately as easy to carry (by weight, at least) as any cell phone, our ability to carry these devices with us remains primitive at best.
So, if we want to see the phone/tablet/entertainment center reach its full potential outside of the coffee table, we either have to come up with a more pocket-sized solution that enables user interaction at the 8-10 inch scale (and some tout Google Glass as the answer to this issue), or we need better ways to carry these devices on our persons.
The demand for surface area is driven both by visibility and by interactivity. People with larger hands, failing eyes, or work that demands higher precision want larger screens with better resolution and more surface area for input. The laser display keyboard, and mobile projectors have gotten some attention in this arena, with power requirements and battery technology matching at a certain critical point. But, the former leaves much to be desired when compared to a touchscreen or even a light, tablet-sized bluetooth or USB keyboard, and the latter has issues with external lighting sources and display resolution. And both of these add extra baggage for the user to carry around. Flexible electronics are underway, but there are years still before we will have flexible touch-screens that we can expand at will.
So, that leaves better ways to carry these devices. If they were not so cumbersome to carry, I don't think most people would object to an eight or even ten inch tablet, especially the super thin models which weigh barely a pound. Even at four times the weight of a cell phone, this is well within the limits for what most people are willing to bear over a long term. But, the traditional backpack is too much, a belt-strapped holster is ridiculous and gets in the way just like the first time this tactic failed with cellphones, and messenger bags still have a limited appeal.