OTOH, carrying around a microphone connected to the Internet which can be remotely enabled at any time without leaving any real trace (battery use/network use is the only real sign, although even that could be covered up to a great degree -- there is probably a way to do either low-fidelity or infrequent audio pickup, maybe keyed on location and charger state, and on-device pre-processing) -- people do this all the time Mostly because there's no real alternative to carrying smartphone yet.
Plus, of course, there's the fact that no modern desktop OS is particularly secure -- either you give up auto-updates and likely fall to bugs, or use auto-updates and are at risk to your OS vendor or anyone who can compel him. So sensors attached to it, as well as stuff processed on it, is also at risk. You can somewhat mitigate this through a large combination of other protections, but it's almost impossible for a single user single machine to solve that problem.
I'd love a custom run of Dell Chromebook 13 or Lenovo Thinkpad 13 Chrome Edition with no built-in mic/camera, and an EPROM vs. EEPROM, and some special case features. Would be willing to commit to buy 10k units at ~$800/unit retail in 8-16GB x 32GB config.
Rdl specified Dell or Lenovo also for the reason that the supply chain for those two ecosystems are well-developed enough that providing customer support won't be a huge hassle.
But yes the question is whether or not that actually exists.