From Gallup's Blog:
Still, Gallup has been studying and investigating the implications of cell phone only households for well over a year now. And, as of Jan. 1, 2008, Gallup has made the decision to include cell phone interviewing as part of the sample used for its general population studies.
This is a significant development, an acknowledgement of sorts that the growing proportion of the population is wireless only, and that telephone samples might not be able to be reweighted sufficiently to account for the skew.
Pollster.com's Mark Blumenthal wrote in 2004:
Still, if not exclusively college age, wireless only adults are predominantly under age 45 (81%). They also tend to live in large metropolitan areas (82%), earn less than $40,000 annually (66%) and rent rather than own a home (62%; the comparable percentages for adults with a landline are 51% age 18-44, 73% metro area, 39%
This profile, Blumenthal suggests, skews Democratic.
There is some evidence that the proportion of wireless-only users jumped dramatically in the past four years, and that might well account for Gallup's decision.