While the two words are obviously related, they carry somewhat different meanings. “Transgendered” implies a process that one went through, such as “married”, “circumcised” or “born again”. “Transgender” implies an inherent attribute, like “male”, “female”, “short” or “tall”.
Most transgender folks think of themselves as always having been what they are, thus for them, the right word is “transgender”.
“Transgendered” could be used to refer to a portrayal of the opposite gender, so (for example) a performer could present a transgendered performance of the opposite sex, even though he or she is not necessarily transgender. Terence Stamp (the transsexual in The Adventures of Priscilla, Queen of the Desert) and Felicity Huffman in Transamerica are not themselves transgender people, but they become transgendered to perform their (transgender!) characters.