IMO, the question about whether "plug computing" will take off in the mass market is still unanswered. There are a few things that the individual can do -- file server, media server, home automation, telephony, as a few examples.
In most of these cases, it is the low power consumption that is the useful feature. If one is serving music/video, it is inefficient to drive the desktop/laptop video as well (the space for such a machine is also a consideration). For telephone, video collection and similar, there is the need to leave the machine on 24X7, so power consumption can be relavant.
In any case, the appeal of any of these functions does not seem to be enough to overcome the resistance by the average consumer. A Plug will have to be much simpler to install and use before the average consumer will buy into the idea.
This situation seems very similar to Linux, about 10 -15 years back -- only the techies would dive into that pool. So, we'll have to wait to see whether Plugs remain in the "environment" long enough for the use to be simplified, and for compelling applications to be created.
One possible future, might be a trend towards less-powerful desktops, which make use of a Plug for some functionality. Those who use/need powerful graphics engines (gamers, for example) won't follow this path. But, someone who mostly uses email and does some Internet browsing, may end up with their actual email on a Plug. . . . IF
, Plugs can be made to be easy and reliable. But, of course, existing paradigms are difficult to overcome, so there would be resistance.
The emerging ideas towards decentralization of services is an interesting trend that may strongly influence the direction of Plug computing. Another user gave the link: http://wiki.debian.org/FreedomBox
Now, if society in general continues to grow repulsed from the corporations that use "free" services to track their activities ["spy"], the all sorts of distributed applications might develop, maybe encryption [as a standard] will give better security.
For now, I'd say a person needs some sort of idea of that
he wants to do with the Plug, and the knowledge and determination to carry it through. In the future, who knows?
Just some thoughts.