There's already ethical questions with regards to external technology, internal will only compound this issue.
To take the fitbit as an example, there's already been at least one case where the data collected was useful in providing medical care that saved life, however the data collection policies of the associated services are such to represent a breach of privacy on an epic scale. All of the data collected has potential for use in an abusive capacity, not to mention rights to this data are erroded completely. It gets even worse if you allow them access to docile media accounts, obviously.
There's also legitimate security concerns with such devices - exploits already existing to convert movements into keypad presses when inputting PIN numbers, typing on a keyboard etc therefore providing a keylogger, the acellerometer is sensitive enough to take the raw data feed it through an algorithym and pull back human speech from the vibrations, making it a portable listening device etc.