According to the NY Times there is an App called "Autography" proposed for the iPad where a digital photo of the author with a fan is sent to an iPad, complete with message from the author written with an electronic stylus, ("Would You Sign My Kindle?" By STEPHANIE ROSENBLOOM, April 13, 2011).
That all sounds a bit complicated. I suspect that a busy author will
instead have an assistant type a message to the reader. That can be overlaid on a stock photo with autograph and sent to the reader's
device. One simple would be by generating a QR code on a screen, or
printed out. The fan would photograph that with their iPad, iPhone,
Android or other device. This would have the advantage that no complex electronic connection would be required.
I used a QR code on a conference poster last week and it worked well. People came up and photographed it for mode details. But no one asked for my autograph. ;-)