I got my Kindle PPW this week. As I was using a DX I couldn't use the sync feature for third party books between it, my iPad and iPhone.
But since getting the PPW I've semi-retired the DX. As it's a WIFI model there's no Whispernet involvement.
I experimented with Calibre by sending a particular book to the PPW, the ipad and the iPhone. What happened was there ended up being 3 copies of the same book in the Amazon "cloud".
So, I did what one of the posters above did - send it to one address, typically the one for the PPW. Once in the cloud, I opened the iOS Kindle apps, tapped on "Cloud", found the book and downloaded it to each device.
I've tested the last read pointers on each device and it updates to all of the others.
I only wish that the DX had WIFI so it could use this service.