I recognize that this is not an Academy diploma, but it is still from the Academy and very similar in form, so I thought I'd post it.
Jean-Luc Picard's Academy Marathon Certificate
Interestingly, it's very similar, but different, from the version found in his scrapbook from Generations.
This is from display at the Star Trek Experience in Vegas. There is a very large version of this image accessible here: Flickr Image