Technological progress has tended to result in miniaturization. While courtesy can be demanded with something obvious like Glass and bluetooth headsets, the same cannot be said for pen recorders or other extremely inconspicuous recording devices that current technology has already made possible. Things that were once only within the realm of Mission: Impossible and government-funded espionage are becoming commercially-available everyday items. It's only a matter of time before we see (hear!) headsets the size of a tiny in-ear hearing aid. What happens when technology advances to the point that we have Google Contact Lenses? Or even ocular implants? Then, how will one know whether they are being recorded?
Since long before Google Glass was conceived, (nearly) every human has come with a built-in, always-on, inconspicuous recording system. Whenever you're within sight or hearing (or even smell) of another, you are - not may be - being recorded. The difference is technology allows for a much more reliable replay function (as opposed to the human memory, which for most people is not all that great). Sooner or later we will have to come to terms with that fact.
Given how readily people exchange privacy (and even relative anonymity) for security/utility/titillation/social chatter, I expect society will become desensitized to omnipresent recording long before personal always-on artificial recording devices become ubiquitous. But I don't see that as necessarily a bad thing. I'm optimistic that we will endure and even manage to flourish as we explore the technological frontier.