This has been all over the internet the past few days - so given my long history working for Disney and that my wife is a development executive at Disney it seemed almost an obligation that I should post it here.
I'm fairly certain that this letter is authentic--- however, even if it's not, the substance of it is. Back in Disney's Golden Era of animation Disney had only men as animators, the women were relegated to inking and painting the animation cels. And look at this stationary! Man, it's frickin' gorgeous!
I did a little online research attempting to track down the source for the letter and whether or not it was genuine. Snopes.com (The Internet reference source for urban legends, folklore, myths, rumors, and misinformation) has no entry for it, however there is a discussion page for it and a link to flickr where the letter is posted with the following explanation: