I've said before that I do not believe that Amazon will ever offer no cost/low cost "upgrades" for paper book owners to ebooks, mostly because
1/ no financial gain
2/ they are not the publisher, and do not own the rights to do this.
O'reilly is a publisher, and they do have an opportunity for financial gain.
When I got this book (The ScreenOS cookbook), the PDF was available after registering, and now they have an upgrade path to get this in all digital formats they provide (pdf, ePub and mobi) for $4.99.
- Ed
edit: If anyone here works with Juniper firewalls, I do highly recommend this book.