As pointed out by Heresy Corner,
The problem isn’t that he called her a bigot; it’s that he assumed, on so little evidence, that she was.
It’s worse than that, of course. This is the same party with a ‘tough’ immigration policy and that famously stole the BNP’s “British Jobs for British Workers” slogan to pander to, well… bigots.
How are we supposed to react to this apparent world of difference between Gordon’s personal feelings, his willingness to be very nice and friendly in public with a woman he privately believes to be a bigot, and the policies he puts forward?
Is the real significance of this event that it’s exposed a bit of a liberal streak (in the ‘Liberal Conspiracy‘ sense of course) in Brown, one that he hides to create an image of himself as a hard nosed anti-immigration populist? Surely it’d have been worse if he’s said, “That was brilliant. She was anti-Immigration and she supports our policy”?

Sod Tebbit. There’s only one story that’s going to dominate the next few news cycles and that’s the footage of Brown chatting to a voter then getting into his limo with his microphone still on, creating an almost pure “The Thick Of It” moment. Transcript:
Brown: That was a disaster… should never have put me with that woman. Whose idea was that? It was Sue I think… just ridiculous (punches back of seat?). Another voice: What did she say? Brown: Och, everything, just some awful, bigoted woman. Used to be some Labour voter… (breaks up)
What effect will this have on the election? I think you can safely predict that the Gillian Duffy herself will have the entire media bearing down on her to give her reaction, and if the Tories and Lib Dems have any sense at all they’ll simply let this one play out without getting too involved.
Another consequence might be to reignite the stories about Brown’s temper and lacklustre social skills highlighted in Rawnsley’s book, “End of the Party”. Is this bad for Labour and Brown? Well, yes. It’s absolutely terrible and sensational at the same time. I’ll take my hat off to Brown though – he’ll have finally put Labour in top billing in the news for the first time this election.
UPDATE: I hadn’t finished typing this up when Brown had already gone on the radio to apologise. Gillian Duffy is apparently, “very upset”. Sky’s Adam Boulton says, “I’d be surprised if any of this turns out to be a vote winner.”