I can ‘sign’ Kindles thanks to Authorgraph.com (was formerly kindlegraph.com). Full instructions are on the site, but I would just add that if you put in a request for an authorgraph from me, please do leave me a message with your name so that I know who you are and can personalise my reply.
Once I have fulfilled the request, the authorgraph pops up as a PDF document on your Kindle next time you sync your device.
Click on this link, it will take you direct to my author page. Alternatively, go to the main page and you can search to see if your favourite authors are on there too!