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When is it legal to to take pictures of people without their knowledge and post those pictures online?

That question is apparently at the heart of an interesting legal case being watched by cyber libertarians.

The case stems from a video artist who surreptitiously took pictures of Apple’s customers using Apple’s own computers and then posted them online at his Tumblr site, “People Staring at Computers.”

He now is apparently under investigation by the Secret Service, according to several news reports  including those from the BBC and Mashable, and also according to the artist’s own Twitter feed.

McDonald apparently went to Apple stores in New York and put programs on the computers. In turn, the web cams on the computers took pictures of customers as they looked at the computers and then sent them to McDonald. McDonald said that he had permission from the stores security guard to take pictures and argued that the pictures were legitimate because they were taken in a public place.

But others have questioned whether an Apple store really is a public venue, in the same way that a street is a public venue. For instance, Google has come under fire for posting pictures on its Street View service taken on the street.

This all began last month. McDonald’s goal, according to Mashable, was to capture “people’s expressions as they stare at computers.”

“I thought maybe we could see ourselves doing this we would think more about our computers and how we’re using them,” he told Mashable.

But last week, just days after making the project public, McDonald apparently came under investigation by the Secret Service.

“@secretservice just stopped by to investigate peoplestaringatcomputers.tumblr.com and took my laptop,” McDonald tweeted Thursday and warned people: “Please assume they’re reading any emails you send me.”

(That singular tweet incidentally had been retweeted some 84 times as of Monday morning.)

A series of tweets followed, including this: “warrant says (peoplestaringatcomputers.tumblr.com ) violates 18 USC section 1030. if you’re familiar w this law, contact me. i just asked @EFF for advice too.

The Daily Dot has calls into the Secret Service and reached out to McDonald. No comments so far.

But McDonald said in a subsequent tweet that he was “encouraged” by the Electronic Frontier Foundation “to stay quiet for now.”

An EFF spokesperson said Monday that her organization could not comment on the case.