In H1 2009 22.7% of US households had a mobile phone but no landline

"In the first 6 months of 2009, more than one of every five households (22.7%) did not have a landline telephone but did have at least one wireless telephone. Approximately 21.1% of all adults--approximately 48 million adults--lived in households with only wireless telephones; 21.3% of all children--nearly 16 million children--lived in households with only wireless telephones.
The percentage of households that are wireless-only has been steadily increasing. The 2.5-percentage-point increase from the last 6 months of 2008 through the first 6 months of 2009 is nearly equivalent to the 2.7-percentage-point increase observed from the first 6 months of 2008 through the last 6 months of 2008. The percentage of households that are wireless-only increased by approximately 5 percentage points in just 12 months, from 17.5% in the first 6 months of 2008 to 22.7% in the first 6 months of 2009."
Source: Preliminary results from the January-June 2009 National Health Interview Survey (NHIS), 16th December 2009
Earlier - The figure was reported as 18% for the same period in 2008