But that seems to be changing with Glass.
Google just released documentation for the Mirror API, which developers will have to use in order to write apps and services for Glass.
But as Jeff Blagdon of The Verge notes, Google's terms of service restricts developers from publishing or distributing their app "outside of the official Google-hosted Google Mirror API Client distribution channel, unless otherwise approved in writing by Google."
So it sounds like Google is taking a page out of
The TOS also prohibit developers from charging for their Glass apps, as well using as ads.
That means, at least for now, once you fork over $1,500 for Glass, you won't have to pay for anything else.