Despite tiny screen sizes, cell phone users spent nearly $400 million on pornographic pictures and video in 2004, an amount expected to reach more than $5 billion by 2010, a recent survey found.

"In 2010 we estimate that expenditure on mobile adult content will represent just 5 percent of total end-user spend on mobile content services," said analyst Nitesh Patel. "We expect services that are built around sports, music and media to perform better, because they appeal to a wider audience of users," he added.

With advancing cell phone technology, one in every two phones sold in 2005 have bright color screens, which will rise to four out of five by 2010.

In the meantime, content providers such as Playboy and rival Private Media Group have begun implementing strategies to ensure subscribers accessing its X-rated material are over 18. Anecdotal evidence shows that interest is high among consumers, with South Korea's SK Telecom reporting in late 2003 that 23% of its traffic over high-speed mobile networks accounted for adult content.