"People have started to see that it's worth the cost," Wu said.

Typical uses of smartphones are also on the rise among people in this income group, according to ComScore. The number of people in this group who listened to mobile music has been growing at about 5 percent, versus a drop of 0.3 percent for the mobile market overall, the company said. Using a browser to check out news or other information also rose 5 percent in that group, compared with 3 percent for the overall market.

iPhone purchases also are rising rapidly in households making between $50,000 and $75,000 per year, and among people older than 45, ComScore said. Increasingly, people in all these groups find it's expected that they'll be able to get e-mail, access the Internet and use other data services on their phones, Wu said. In its studies of the mobile market, ComScore uses a sample of more than 33,000 users.

Wu doesn't expect the current economic downturn to make much of a dent in this trend, which follows at least two years of rising monthly spending on mobile services.

"I don't see there's going to be much of a slowdown, just because wireless devices are so much more of a necessity than they used to be," Wu said.