This may not be the case this year (perhaps because the electorate is suddenly so polarized by age). Nate Silver at Fivethirtyeight did a little study comparing the polling organizations that use cell phone numbers with those that do not. It's a small-N study, but it's still pretty suggestive:
Six of the seven cellphone-friendly pollsters have had a Democratic (Obama) lean, and in several cases it has been substantial. On average, they had a house effect of Obama +2.8. By comparison, the control group had essentially zero house effect, so this would imply that including a cellphone sample improves Obama's numbers by 2.8 points. (Or, framed more properly, failing to include cellphones hurts Obama's numbers by 2-3 points).Just to be clear: these findings suggest that those polling firms that do not survey cell phone users are understating Obama's true support by 2-3 points. This is a huge difference in a close election like this one. In Silver's state projections, he finds that accounting for cellphones moves Florida from lean-McCain to tossup, and moves Nevada from tossup to lean-Obama.
The difference is statistically significant at the 95 percent confidence level. Perhaps not coincidentally, Gallup, Pew and ABC/WaPo have each found a cellphone effect of between 1-3 points when they have conducted experiments involving polling with and without a cellphone supplement.