Well, how do you sign an ebook?
There's something called http://www.authorgraph.com/
which I used once just to see how it worked. I got a one page pdf that has the cover page of an e-book with what is supposed to be K.W. Jeter's signature on it.
Not exactly the same thing but my way of getting e-books signed is handing the author a Sharpie along with my Kindle. Works every time.
After the first author, Cherie Priest at the bottom, signed it I kept smudging the signature no matter how I tried to hold the Kindle when reading so I just got a cover for it to protect the back more than the screen. I don't actually read much from the Kindle itself any more since I got my Android phone so it's pretty much an autograph collector at this point.
The authors who have signed it are:
Robert J. Sawyer
Catherynne M. Valente
Mary Robinette Kowall