His doubleTwist software allows people to copy songs from their iTunes library on to non-Apple devices.

"The Cure for iPhone Envy. Your iTunes Library on any device. In Seconds," the five metre-wide ad read.

Johansen intended for the ad to go up on Monday to coincide with Apple's Worldwide Developer Conference, where the company unveiled a host of new products including a new iPhone model.

But the poster, which appears to be right outside the store but is actually plastered besides a train station exit, was put up by the firm handling the advertising on the Friday before the conference, three days earlier.

Within a day, the ad was removed. Bay Area Rapid Transit (BART), which operates the train system, said it pulled the ad down because it was "too dark" and did not let enough light through.

Johansen rejected this justification and published pictures on his blog showing "plenty of light coming through the ad", but tried to appease the ad agency by submitted new creative that had a white instead of black background.

Again, the ad was rejected, "for having a solid white background".

"Apple is a major BART advertiser (in the past they've plastered entire BART stations with iPod ads)," Johansen.

"It's pretty obvious what's going on here... I'm sure our ad will conveniently be back up after WWDC ends [on Friday]."

By the middle of this week, after extensive publicity from technology publishers, the ad was back up, after Johansen settled on a transparent background. He plans to leave it up for months, barring any further setbacks.

Johansen shot to worldwide fame and notoriety when, at the age of 15, he cracked the encryption scheme used by DVD.

This allowed for DVDs to be copied and played back on any device, sparking a flurry of legal action from the film industry. But Johansen was never found guilty.

In 2006, he claimed to master the inner workings of the iPod and its FairPlay encryption technology, allowing him to remove many of the restrictions Apple places on its users.

Songs purchased from Apple's iTunes store can't be played on non-iPod devices, and, if you've bought songs from other music stores, the chances are you won't be able to play them on the iPod either since they use a form of copy protection that Apple doesn't support.

Through doubleTwist, Johansen has developed programs to get around these restrictions, and plans to license them to digital music stores that are looking to sell copy-protected songs capable of being played on the iPod.

The company counts Hon Kong billionaire Li Ka-shing and former Disney boss Michael Ovitz among its investors.

Johansen has said his driving force is a belief that users have the right to listen to songs they have bought legally on any device they own.

He hopes his program will also replace the proprietary software handheld gadget makers bundle with BlackBerry and Nokia phones to allow people to transfer music and video from their computer.

Unlocking the iPod-iTunes ecosystem is seen by many as a good thing for consumers, as it will most likely result in increased competition to the iTunes Store, possibly resulting in lower prices and a higher quality service.