One out of every three mobile users who remembered seeing a mobile ad said "they responded to it in some way," while 50 percent of iPhone users said the same, according to fourth quarter 2008 research by mobile social network Limbo and market research agency GfK NOP, eMarketer reported.

According to the research, "iPhone users are than twice as likely as non-iPhone users to browse the mobile Web and more than three times as likely to use location-based services."

The study indicated that most ads viewed both by all mobile users were SMS messages, according to eMarketer.

Rob Lawson, Limbo's CMO and co-founder, told eMarketer that as more and more people move away from SMS-only handsets, the gap between iPhone and non-iPhone users will narrow.

Mobile ad spending is expected to increase by 40 percent to US$2 billion this year, and to $2.9 billion in 2010, according to JPMorgan's report "Nothing But Net," eMarketer reported.