A Forrester Research study found that iPhone users are “more than twice as likely to access the Internet from their phone as working BlackBerry, Palm, or Windows Mobile device owners.”

The research suggests iPhone users are a peculiar class of smartphone customers. iPhone users are younger, and perhaps even more productive at work than those who use competing smartphones. They are more educated and affluent: 49 percent of iPhone users have a college education, and 67 percent earn more than $70,000 a year – and iPhone customers spend more on their service then other mobile phone customers. The data was compiled from 32,228 working U.S. adults in 2008. It found that those who own an iPhone are more active on their phones and more connected to the internet than those who fell into generic “smartphone” or “mobile phone” categories.

Those who carry an Apple iPhone are more likely to stay connected to their employer’s network. Workers with an iPhone also usually leave their laptop at work, suggesting the iPhone replaces a traditional notebook. This may be the reason why when comparing customer Internet usage, the study shows that the iPhone blows away its competitors.

When the study started, the iPhone had only been available for six months and its starting price of $500 was considered by many to be prohibitively high. With Apple announcing this week a $99 iPhone 3G (subsidized by carrier AT&T) the phone is expected to have even more mass-market penetration. Forrester Research, based in Cambridge, Mass., plans to re-visit the study this year to see how the iPhone’s user demographics may have changed.