The chart at right says it all.

Starting around Nov. 28 — six days after Samsung began airing its Galaxy Nexus ads mocking Apple’s (AAPL) customer base (see here) — the Korean conglomerate’s “buzz” started rising and the iPhone’s drifting down.

And on Wednesday, according to a press release issued by YouGov, something unthinkable happened:

“Samsung has just edged past the iPhone in consumer perception in the US (adults 18+), likely powered by their new set of ads bashing the Apple fanboys who camp out for hours to buy the new iPhone.”

YouGov’s Brand Index tracks the public perception of some 1,100 consumer brands — including Apple — by interviewing 5,000 people every weekday and asking them “If you’ve heard anything about the brand in the last two weeks, through advertising, news or word of mouth, was it positive or negative?” Scores can range from 100 to -100.

The recent shifts in the public’s perception of Apple and Samsung, according to YouGov, were driven mostly by adults aged 50 and up.

This is not the first time Apple’s brand has taken a beating in YouGov’s index. In the spring of 2010 — at the height of the Gizmodo affair, the Flash brouhaha and a Jon Stewart take down, its score fell from a high of 80.2 to a low of 66.1, prompting the New York Times to ask “Has Apple lost its cool?“