The latest US Centers for Disease Control (CDC) report on "wireless substitution" (for landlines) finds that "26.6% [of US homes] had only wireless telephones . . . during the first half of 2010. In addition . . . 15.9% received all or almost all calls on wireless telephones despite having a landline."
What this means is that 42.5% of US homes are mobile only or mobile centric. For the 16% that relies primarily on mobile phones, the landline has become a kind of "spam catcher" for unimportant calls.
Landline abandonment is not correlated with income status according to the CDC; "not poor" was the group that had the largest percentage of non-landline users. Note in the second graph below, 53% of those "not poor" have abandoned landlines.
There are all kinds of implications for market research and political polling, as well as telephone sales and marketing. Indeed, "robo calling" is not permitted to mobile handsets under law. Humans must initiate calls to mobile phones, making them much more expensive.