And, I can tell you from personal experience that this works. I've e-mailed 200+ books I have from other places (Smashwords, Baen, Google, Mobileread, etc) to my Kindle. When I open the book and read for a bit, then open the same book on my iPhone or the iPad - I am offered the chance to go to the furthest read point, just as if I had purchased the book from Amazon.
Kindle Personal Documents Service makes it easy to take your personal documents with you, eliminating the need to print. With Kindle Personal Documents Service you and your approved contacts can send personal documents wirelessly to your supported devices, archive them in your Kindle Library and download them any time you have a wireless connection. Just as with Kindle books, Whispersync automatically synchronizes your last page read, bookmarks, and annotations for personal documents (with the exception of PDFs) across your supported devices.
You can view and download archived personal documents from your Kindle Library on Kindle Keyboard (with the latest software update, version 3.3), Kindle, Kindle Touch, and Kindle for iPad, iPhone and iPod touch (running Kindle app version 2.9 or greater). Kindle Personal Documents Service is currently not supported on other free Kindle Reading Applications. Whispersync of your last page read, bookmarks, and annotations is available on your archived personal documents that have been converted into Kindle format.