The latest numbers on wireless only households as measured by the NHIS have just been released. They now estimate that as of December 2008 20.2 percent of US households have only a wireless telephone. The 2.7 percent increase in the second half of 2008 is the largest six month increase since the NHIS has been collecting these data.
I recently saw some estimates from SNL Financial that appear to have taken the NHIS trends and projected them forward. Their data say that by 2013 a whopping 37 percent of US households will be wireless only.
As I have noted in the past, the most statistically robust way of dealing with this problem is to spend the extra money and dial some cell phones. The financial hit is not too bad as long as you only have to do limited dialing. But as the proportion of cell only Rs increases, so do the costs. While there is always the option of mixing in wireless only Rs from an online panel that is a journey into a statistical no man's land.