Some more observations about the Paperwhite, from someone who doesn't use USB-
Sending mobi files via email, it doesn't matter what you do (or don't) enter for "Personal Doc Tag" when converting. Files will always appear on the Paperwhite with the "Personal" banner across the cover when sent via *kindle.com addresses.
Sending azw3 files via email doesn't seem possible at all. Always get a bounce from Amazon: "The following document, sent at 05:47 AM on Thu, Oct 04, 2012 GMT could not be delivered to the Kindle you specified" Gotta stick to mobi.
They also seem to have done away with the dotted progress bar/chapter markers at the bottom of the screen. That's not something Calibre can help with though!