Is your laptop a fancy piece of luggage or an extension of your mind? That's the central question facing a federal appeals court in a case that could sharply limit the government's ability to snoop into laptop computers carried across the border by American citizens.

The question, before the 9th U.S. Circuit Court of Appeals, arose from the prosecution of Michael Timothy Arnold, an American citizen whose laptop was randomly searched in July 2005 at Los Angeles International Airport as he returned from a three-week trip to the Philippines. Agents booted the computer and began opening folders on the desktop, where they found a picture of two naked women, continued searching, then turned up what the government says is child pornography.

In June 2006, a judge from the U.S. District Court for the Central District of California threw out the evidence, finding that customs officials must have at least "reasonable suspicion" to begin prying into the contents of an electronic storage device, a decision the government is now appealing.

"Electronic storage devices function as an extension of our own memory," Judge Dean Pregerson wrote. "They are capable of storing our thoughts, ranging from the most whimsical to the most profound. Therefore, government intrusions into the mind – specifically those that would cause fear or apprehension in a reasonable person – are no less deserving of Fourth Amendment scrutiny than intrusions that are physical in nature."

While it's not clear how many laptops are searched at the border each year, both business and recreational travelers are increasingly toting computers with them, complete with hard drives full of personal pictures, confidential corporate documents and revealing internet logs. An October 2006 survey of business travel executives revealed that some companies were rethinking rules on proprietary information being stored on traveling laptops, and 1 percent of the respondents reported they had, or knew someone who had, a laptop confiscated at the border.

The reach of such searches will likely widen as more and more people opt for smartphones, such as Apple's upcoming iPhone, which combine elements of traditional computers with the voice capabilities of a cell phone.

The California decision is the first to challenge that trend, and it makes laptops, and even USB memory sticks, very different from every other item brought across the border, including luggage, diaries, prescription drug bottles and sexual toys – all of which customs and border agents have been allowed to search without cause for years under the "border exception" to the Fourth Amendment.

The government says the rationale behind that exception – that border agents are responsible for protecting the safety of the nation and enforcing copyright and obscenity rules – logically extends to laptops. "For constitutional purposes nothing distinguishes a computer from other closed containers used to store highly personal items," the Department of Justice argues in its appeal brief.

Moreover, requiring government agents to have a reasonable suspicion before searching a laptop will invite smugglers and terrorists to hide contraband and evidence there, the government argues. "If allowed to stand, the district court's decision will seriously undermine the nation's vital interest in protecting its borders by removing the significant deterrent effect of suspicionless searches," reads the filing.

Arnold's lawyers, Kevin Lahue and Marilyn Bednarski, disagree, arguing that it's not very difficult for law enforcement agents to come up with "reasonable suspicion."

"No ordinary traveler would expect their private files to be searched at the border without any reasonable justification," they told the appeals court. "The government's argument that a traveler can simply avoid exposure by leaving the laptop at home is an oversimplification of its function and role in daily life."

Lahue has support from the Association of Corporate Travel Executives and the Electronic Frontier Foundation. The two groups submitted a friend-of-the-court brief Tuesday arguing that suspicionless searches of laptops are overly invasive, and that prior to the California ruling, the government had no limits on what it could do when it seizes a laptop and makes a copy of the hard drive.

Already travelers have reported customs agents seizing laptops, making copies of the hard drive and returning the computers weeks later. That practice scares the travel execs' association and the EFF, which argue that under the government's reasoning, border authorities could systematically copy all of the information contained on every laptop computer and cell phone that crosses the border, without any court oversight.

"A suspicionless unrestricted search of a laptop computer is simply electronic eavesdropping after the fact," the groups told the court. "(It) is distinguishable from the forbidden general searches of Colonial times only by the technologies involved."

The case's outcome is far from clear-cut, according to Lahue.

"A lot will depend on whether the court decides it's like searching a piece of luggage or like a body-cavity search," Lahue told Wired News. "A diary, even one that is labeled 'my secret sexual fantasies,' has always been fair game."

The government's reply brief is due June 26, and the case will likely be argued sometime in the fall.