U.S. mobile advertising will quadruple between 2009 and 2014 as mobile-oriented marketing efforts catch up with mobile use and traditionally conservative industries start trying to reach users of Apple's iPhones and other smartphones, according to Jordan Rohan, managing director of equity research, Internet and digital media at Thomas Weisel Partners.

And that's his conservative estimate.

U.S. companies, which spent about $500 million on mobile marketing and advertising last year, will boost spending by more than 60% this year to about $820 million, Rohan said at the digiday:MOBLE conference in Los Angeles today. Annual mobile-advertising spending will surge further to almost $2 billion by 2014, according to what Rohan called his "base" estimate, and to more than $4.3 billion "if everything goes right," Rohan said.

Mobile-advertising spending will start matching the growth in mobile-Web usage as industries not traditionally associated with the practice, such as financial services and pharmaceutical, boost their marketing spend in an effort to reach a U.S. mobile-Web user base that's jumped 24% to 74 million within the past year, Rohan said. U.S. companies spent $80 million on mobile advertising in the fourth quarter, which Rohan referred to as "a watershed moment."

"We're heading toward (mobile advertising being) a line item and away from the random experimentation stage," said Evan Neufeld, vice president of marketing at mobile-Internet research firm Ground Truth, who was speaking on a panel with Rohan at today's event.

Amid such growth, challenges within the industry will include properly targeting mobile-device users whose spending habits diverge wildly, according to Neufeld, who was a founding analyst of Jupiter Research's digital-advertising practice.

Additionally, much of mobile marketing's profitability growth will be dependent on whether Apple's iPhone emerges as the primary smartphone platform for mobile marketing, or whether competition from Google's Android creates an open platform and prevents a dominant player that will create what Rohan called "not a very healthy ecosystem."

Still, with mobile searches exceeding online searches during most weekends, according to panel speaker William Hsu, chief product officer for AT&T Interactive, there is room for many mobile-marketing players, according to most of the panelists.

"We're still very early in the days of mobile targeting," said panelist Krishna Subramanian, co-founder of mobile-advertising exchange network Mobclix. "Expect all digital buys to have some sort of mobile component to it."