These are some fascinating numbers from Pew:
There's a huge difference when cell phones are included in the sample -- on average about a 4 point net swing towards Democrats. Given that most pollsters don't contact cell phones -- doing so is quite expensive -- Democrats could easily outperform their polling average based on the cell phone bias alone. The reason, according to Pew, is that one in four American households is cell-phone only and that voters who live in these households are more likely to vote for Democrats.
Also worth noting: while Pew still shows the GOP with a 7 point lead among likely voters even when including cellphone results, those numbers are from late August and early September when the GOP was at its peak.