Actually, it was talked about ad nauseum on FR in the runup to the 2008 elections. Many FReepers placed desperate hopes that it would materialize... it didn't, and it won't now. The "Bradley Effect" is a mirage.
It wasn’t alot (probably a few points) but enough to affect close election like this maybe.
How so? The Bradley Effect is an against vote, not a for vote.
In 2008, the vote was Anybody But McCrisis, so the vote was a thumbs down on Bush proxy McCrisis while taking the Unknown Quantity/Acceptable Challenger, plus bonus points for voting for a black guy.
This vote is Anybody But Obama. Unknown Quantity is gone, Americans know exactly who Obama is. Californians knew exactly who Bradley was, going into that gubernatorial race, they were going to vote Deukmejian, but they weren't going to tell pollsters that they were going to vote against Bradley.
Is it 5 points? No. Could it be 1-2 for Mittens? Entirely possible.