Pleaded is the standard past tense and past participle of the verb plea. Pled has always been considered incorrect by people who make such judgments, but it is so common that we have to accept it as an alternative form. And pled is not just an Americanism, as some have claimed. It appears just as often (about one pled for every twenty pleadeds) in current British and Canadian news publications. Australians are the exception; they still seem to shun pled almost completely.
Pled is fairly common in American, British, and Canadian popular usage. Here are a few examples out of many:
Eighty-two confessed, while half a dozen others pled the Fifth Amendment, which protects people from being forced to incriminate themselves. [Washington Post]
So too did the Miami-based accountant … who pled guilty to helping forge documents. [Financial Times]
A Kelowna Mountie has pled guilty to a pair of charges stemming from a violent domestic dispute. [Vancouver Sun (article now offline)]
But because pleaded is much more common and is unanimously recommended by English authorities and reference books (the dozen or so we checked, anyway), it is safer than pled.
The ngram below graphs the use of pleaded and pled in English-language books published from 1900 to 2000. It shows pled slowly gaining ground but still far behind pleaded.