Early symptoms: air conditioning, vacuum cleaner robots, zero-effort TV dinners, a sterilized environment (eg using UV), firewalling/spam-filtering services, Everquest.
Later symptoms: development of Culture style AIs (probably from spam filters and certain non-player characters in games).
Our interactions with others and with the world become highly mediated. The environment becomes highly controlled.
This is certainly a technological option. I suspect it is suicide.
Alternative scenario: Augmentation. We create tools -- effectively extensions of ourselves -- that allow us to live in more and more hostile environments.
Early symptoms: widespread popularity of high tech hiking gear, disadoption of air conditioning and heating in favour of better clothing, pharmaceutical brain enhancement, social-space visualization tools.
Later symptoms: brain augmentation, ability to interact with the world at a nano scale, conscious control of the immune system, space travel, loss of humanity.
One aspect of this scenario deserves special mention, as its extreme form is almost a scenario of its own: disintermediation. Wide band social interaction, leading to group minds.
Early symptom: mobile phones.