More than 20 million United States homes rely solely on mobile phones instead of landlines, according to new research by market researcher Nielsen Mobile.

This means that 17 percent of wireless subscribers use their mobile phone as a substitute for a landline connection. The research from Nielsen Mobile, a service of Nielsen Co., suggests that one in five U.S. households could be wireless-only by the end of 2008.

"For marketing of mobile, [this] represents an opportunity to expand the power of the bundle," said Alison LeBreton, vice president of client services at Nielsen Mobile. "For marketing on mobile, this means the opportunity to lean into an even closer relationship between a customer and their phone."

The Nielsen Mobile report titled, "Call My Cell: Wireless Substitution in the United States," talks about the fact that consumers are cutting the cord on their landlines and why.

Per the report, the reason that many consumers are cutting landlines from their lives is because the U.S. economy is tightening. One less line could save the average consumer about $40 per month.

U.S. cord-cutters tend to have lower income-levels -- 59 percent have household incomes of $40,000 or less.

Their households are smaller as well. With just one or two residents, they are more likely to cut the cord than larger households.

Additionally, moving or changing jobs are the biggest life events associated with cord-cutting.

In fact, 31 percent of cord-cutters moved prior to cord-cutting and 22 percent changed jobs.

These cord-cutters tend to use their mobile phones more than their landline peers - 45 percent more per phone - but still save an average $33 per month in a household of one subscriber.

"Wireless substitution requires a certain commitment to the role of mobile in your life and it fosters ubiquitous connectivity that expands the opportunity for mobile marketing," Ms. LeBreton said.

"We know that subscribers keep their mobile phones very close to them already, but how much more so when it is in fact their only telephone connection," she said. "That closeness represents an increased engagement potential for mobile marketers."

Wireless substitution doesn't work for everyone.

Ten percent of landline phone customers have experimented with wireless-only in their household, but then returned to landline service.

Nielsen Mobile reports that needing a landline for another service (security system, satellite TV, pay-per-view and fax machine) is the primary reason people mend the cord.

Will carriers be able to capitalize on this trend?

Ms. LeBreton said that carriers have proven they can capitalize on consumer trends, and wireless substitution will be no different.

Cord-cutting expands a customer's reliance on and relationship with a wireless carrier -- it becomes less appealing to switch carriers when they don't have a temporary backup in landline.

Carriers also see the close relationship between landline cord-cutting and wireless data and PC cards. The Internet possibly may be the next realm of wireless substitution.

"Wireless substitution is a trend to follow, with important social and business implications," Ms. LeBreton said.

"As consumer behavior evolves rapidly, we'll all do well to monitor this evolution, anticipate changes in the market and somehow capitalize on the ground that moves beneath us," he said.