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).
Source: ayse