As spotted by The Verge, Google has been granted a patent to track "gaze direction information" to see when users are looking at and responding to advertising in the both the real world and online.
US Patent number US008510166, which Google initially applied for in May 2011, details the use of a head-mounted sensor to determine when you look at advertising and for how long.
Not only that, the technology can also be used to measure your pupillary response to the ad and infer an "emotional state of the user while viewing the external scenes based at least in part upon the pupil dilation information". This would, in theory, allow advertisers to estimate a level of how "engaged" the viewer is in the advertising.
The patent further offers the application of a "pay-per-gaze" system, where:
Pay-per-gaze advertising need not be limited to online advertisements, but rather can be extended to conventional advertisement media, including billboards, magazines, newspapers and other forms of conventional print media. Thus, the gaze-tracking system described herein offers a mechanism to track and bill offline advertisements in the manner similar to popular online advertisement schemes.
While the patent application pre-dated Glass, and thus doesn't mention the product directly, it's not unreasonable to assume that the system would be an integrated part of Google Glass and perhaps other wearable computing technologies that may arise.
Don't panic, however; Google has generously allowed for some degree of privacy:
To protect individual privacy, personal identifying data may be removed from the data and provided to the advertisers as anonymous analytics. In one embodiment, users may be given opt-in or opt-out privileges to control the type of data being gathered, when the data is being gathered or how the gathered data may be used or with whom it may be shared.