Here's a presentation I made at the 15th Summer School of Telecommunications in 2006. The subject is "Principles of Ubiquitous Computing".
In retrospect, there are some notes to be made. Back then when I was reading the available literature and research, there was a kind of concensus that the peer-to-peer model of communication - device-to-device communication without intermediaries - would play a big role, as this would let the device deployments scale without requiring new or existing static network infrastructure. However, the bulk of the ubiquitous computing devices of today (sensors, smart phones, electrical consumption readers, etc.) rely on static communications infrastructure to function.
Also, the "Spam/Big Brother Society" is as relevant a danger as then. As I see it, the danger has merely evolved and is even more extensive today.
Today, more and more information about private individuals are collected with the justification of "with the information, we can show you more relevant advertisements". The infrastructure of knowing who you are, what you think and who you know is in place to learn what stuff or services we might be currently missing.
At the moment the Spam Society is very benign. However, once this infrastructure and data is in place, it can be hard to remove it or to escape its reach, or to prevent it from transforming into a Big Brother Society. Even if one were to vanish as the target of the data collection today, the previously obtained information would still contain a lot of data that could be misused.
For example, what can happen if a political party with a violent agenda takes power, one way or another? If your profile indicates you have been thinking wrong thoughts, instead of getting advertisements, you would get night-time visitors taking you for a long car ride that culminates in a neck-shot in the woods. Interestingly enough, there is prior art in this kind of horror scenario: the Nazi government used census data which they data mined with IBM's help to weed out people with Jewish ancestry.
As for the current state of ubiquitous computing devices, the smart phone stands as a lone king. It helps people organize their lives, entertains them, helps them keep connected with others, helps them document their lives with photographs and videos, and so on.
Although not quite as invisible as Weiser envisioned it, for those who have one, the smart phone is always present, ready to serve - and with modern UIs, it tries to not get in the way too much. I'd say at the moment the smart phone is closest to Weiser's vision of calm technology. Also, over time, the smart phone has gotten only better and I expect this trend to continue.
Generally, a big downside I see with all current smart phones is the level of trust that needs to be placed on the maintainers and owners of the smartphone ecosystem to not abuse the data they collect (the location data, contact data, calendar data, etc.).
For example, Google backs up your WLAN passwords if you enable the Backup My Data option. It's convenient in case you lose your phone, but do you know who in the end has access to the data and what they do with it? If you disable the option, the data is said to be removed. Fine; now, how will you know this to be true? You can't know this, there is no way to check, so you just have to have trust. There are technical ways to remove the reliance on trust (e.g. encrypt the backup locally with a user-given key and then upload it), but at the moment such techniques are not used.
That said, I am a happy user of an Android smart phone. Android is open enough and the phone hardware it runs on is documented enough to let a community of enthusiasts make their own aftermarket firmware. Therefore, if I ever become unhappy with the stock Android, I can always install Cyanogenmod.