Porn Valley- Apple Computer took 20 days to reach 1 million downloads of video files from its online store; the Web site SuicideGirls, offering free videos of unclothed models, hit the mark in about a week.

One of the quickest industries to take advantage of the new video iPod, and other new gadgets, is one that has often been at the forefront of other technological innovations: porn.

Pornography is spreading from the computer desktop to the small screen, to pocket-sized devices such as cellphones, digital music players and portable game players. The phenomenon is being pushed not only by alternative-culture sites such as SuicideGirls but also by old-school skin traders such as Playboy and Penthouse, which this month announced plans to offer movies for viewing on portable devices.

Almost as quickly, a trade group for wireless carriers has responded with a plan for a rating system, such as that used in the movie industry, to help prevent children from seeing inappropriate content on mobile phones.

The Cellular Telecommunications & Internet Association last week announced that member companies agreeing to the voluntary proposal will not offer adult-oriented images or programming until such a system is in place.

Though still in its infancy in the United States, the portable porn market could grow to nearly $200 million a year here by 2009, according to research firm Yankee Group.

Harvey Kaplan, an Internet porn-industry veteran, said sales are growing “fast and furious” for his new company, called Xobile, which focuses on selling porn for cellphones and handheld computers.

“The adult Internet marketplace hit a ceiling about two years back,” he said. “I started looking for alternate ways to reach the same market – this seemed the logical place to go.”

The proliferation has caught some off-guard. Parry Aftab, the director of WiredSafety.org, an organization devoted to helping parents keep porn away from their kids online, said the mobilization of digital porn is something even most plugged-in parents generally are not aware of yet.

“Even parents who are watching their kids’ computers are clueless that they’ve put a (cellphone) in their kid’s hands that can be used to trade this stuff around,” she said.

A more fundamental issue might be whether people even want to see pornography on tiny, 2-inch video screens.

Ralph Whittington, a former curator at the Library of Congress dubbed “the king of porn” for amassing a carefully cataloged collection of adult magazines, videos and DVDs, said that he does not get the trend.

“I can’t see anybody wanting to watch while they’re sitting at the airport,” Whittington said. “I just don’t see how it’s going to be all that popular.”

Proponents argue that the anonymity and privacy offered by a mobile gadget could be a major selling point.