According to the Pew Research Center, the differences between voters with traditional landline phones and voters with mobile phones are minimal:
Pollsters are continuing to monitor changes in telephone use by the U.S. public, since most surveys are still conducted using only landline telephones. Growing numbers of Americans are reachable only by cell phone, and an even larger number who have both a landline and a cell phone may be “functionally cell-only” because of their phone use habits. The latest Pew Research Center national survey, conducted June 18-29 with a sample of 2,004 adults including 503 on a cell phone, finds that the overall estimate of voter presidential preference is modestly affected by whether or not the cell phone respondents are included. Barack Obama holds a 48% to 40% lead in the sample that includes cell phones, and a 46% to 41% advantage in the landline sample. Estimates of congressional vote are the same in the landline and combined samples.
The cell-only and cell-mostly respondents in the Pew poll are different demographically from others. Compared with all respondents reached on a landline, both groups are significantly younger, more likely to be male, and less likely to be white. But the cell-only and cell-mostly also are different from one another on many characteristics. Compared with the cell-only, the cell-mostly group is more affluent, better educated, and more likely to be married, to have children, and to own a home.
Finally, a technical note (the thing which interests political scientists):
The more serious challenge to survey research posed by cell phones is the declining absolute numbers of certain types of respondents, most notably the young. In recent Pew Research Center surveys, only about 10% of respondents in landline samples are under age 30, which is roughly half of what it should be according to the U.S. Census. Young voters reached on landlines share many of the characteristics of the cell-only group, especially in terms of political views. That is why statistical weighting of the landline samples helps to correct for the absence of the cell-only. But the shortfall of young respondents in absolute numbers means that pollsters are limited in their ability to analyze differences within this age group.