And this development is not unique to Pew Research.
Sorry, Wrong Number. Surveying Americans by phone is increasingly problematic. As a result, pollsters are trying to incorporate new—but imperfect—alternatives. (National Journal, 7/19/2012)
Some numbers from the article.
34%. Cell-phone only households. (2nd half of 2011*.)
20%. Cell-phone only households. (2nd half of 2008.)
60%. Adults 25-29 in cell-phone only households.
8.5% Adults 65+ in cell-phone only households.
*According to Chetan Sharma (paragraph 4), it's now up to 40% of households.