In a survey of iPhone owners, Rubicon Consulting found that a third of iPhone users carry a second phone in addition to their iPhone. I'm one of those folks: I have a T-Mobile phone that I use for personal calls; I use my iPhone as a business phone and for mobile Web and email. I didn't want to break my T-Mobile contract and I'm happy paying for a second phone to keep work and home separate. I didn't think I was the only one with two phones, but 33 percent seems surprisingly high. Here's something the survey didn't tell you, but you might have guessed: iPhone users have lots of disposable income. More details from the study after the jump.

• The most heavily used data function on the iPhone is reading (but not writing) email.
• More than 75 percent of iPhone users say it has led them to do more mobile browsing.
• About 40 percentof iPhone users say the iPhone has trouble displaying some websites they want to visit.
• About 50 percent of iPhones replaced conventional mobile phones, 40 percent replaced smartphones, and 10 percent replaced nothing. Among conventional phones, Motorola Razr was the phone most often replaced. Among smartphones, Windows Mobile and RIM BlackBerry models were most often replaced.
• 28 percent of iPhone users surveyed said strongly that they often carry their iPhone instead of a notebook computer.
• About half of iPhone users are under age 30 and about 15 percent are students
• The iPhone has increased its users' monthly mobile phone bills by an average of 24% percent, or $228 extra per year.
• Almost half of iPhone users changed carriers when they got the iPhone.
• The iPhone has probably increased AT&T's gross service revenue by about $2 billion per year.

(Photo by AP/Paul Sakuma)