+1 and the wiki actually mentions that theory,

In his book Blink: The Power of Thinking Without Thinking (2005), author Malcolm Gladwell presents evidence that suggests Pepsi's success over Coca-Cola in the "Pepsi Challenge" is a result of the flawed nature of the "sip test" method.[6] His research shows that tasters will generally prefer the sweeter of two beverages based on a single sip, even if they prefer a less sweet beverage over the course of an entire can.[7] Additionally, the challenge more often than not labeled the Pepsi cup with an "M" and the Coca-Cola cup with a "Q," suggesting letter preference may drive some of the results.[8]