Lower-income bracket U.S. users are flocking to Apple’s iPhone, sending an early signal that smartphones are changing from a luxury to a vital tool, according to internet research company ComScore Mobile.

In a group of surveys lasting several months that ended in August, ComScore found that iPhone purchases grew fastest among people with annual household incomes between $25,000 and $50,000 (considered lower to middle class). The growth rate in this group was 48 percent, compared with just 16 percent among people with incomes above $100,000. Many would be surprised by this, seeing the iPhone as more of a high-end luxury device.

Consumers in that lower-income group still made up only 15 percent of the iPhone crowd (not surprising), which is dominated by higher-income males between the ages of 18 and 35, the company said. But the fact that they are increasingly willing to pay $199 for an iPhone 3G and about $70 per month or more for A&T’s service plans suggests smartphones are becoming more of a mainstream product, and less of a luxury for “rich” people, which is usually a good sign for the future of any device in this class.