It appears that in some cases Google will conduct an on-demand fetch of your page to dynamically create an Instant Preview. The on-demand fetch happens when a searcher places his mouse over the search result on Google and the image preview comes up. Some analytics tools, including Google Analytics, may consider that a visit, because Google Instant Preview is actually visiting the page in real-time to get that on-demand Instant Preview.
There is no estimated time for when this may be resolved. So if you have seen a skew in your analytics data in the past two weeks, this may be the reason.
Postscript: The Google Analytics blog posted an update saying they fixed the issue at about 6:30 pm (EST). Note, they will not be reprocessing the data, so if you want to clean up your data, you can learn how to do so over here.