Recently we bought a kindle touch and I discovered that it has a feature where the amazon cloud remembers the last page you are on. This way you can swap between the kindle or kindle app (on the iphone/ipod touch) and start reading where you left off.
The main requirement for this feature to work is that you load your mobi books onto the kindle or kindle app via the personal documents feature of the kindle store. Each kindle device or app you register to the same amazon account receives a unique email address. You can then send a ebook to the device by sending it to its specific email address.
To get the sync feature to work you need to send a single email to all the devices you want to be able to synchronize across.
I haven't been able to configure calibre to send an email to multiple addresses. I've tried using spaces( ), commas(,) and semi-colons(;) to delimit multiple email address but they don't seem to work.
The method I've found is to install hmailserver and the configure it as follows:
1) turn of auto-ban
2) in "IP Ranges" turn off all authentication for "My Computer"
3) in rules create a "Kindle Group" which matches for your outbound email address configured in calibre and forwards emails to all your kindle devices/apps
Then in calibre, set the email preferences as follows. Make sure of the following:
a) your send email address is setup in Amazon "Manage My Kindle" page as approved personal document email.
b) outbound email address is for your first kindle device and matches step 3 above.
Now when you hit send book via email in Calibre, hMailServer will automatically send copies of this email to all your other kindle devices.