Are all the messages kept on the phone?
How does it relate to the Whisper Systems end to end encryption on the Android to Android version of WhatsApp?
Is there some relationship between the two - is this very bizarre web interface there because of constraints relating to privacy and encryption?
i.e. WhatsApp don't have your messages.
[Edit] Is it peer to peer in some fancy way - is it only for Chrome because it uses WebRTC or something?
It seems to be trying to keep the link between users and phone numbers as 1:1 as possible. This has a number of effects against bots and spam, ties in with matching users from other user's addressbooks etc. It's probably also bad for privacy/pseudonymity.
Might have been in this video [1]... Not sure at the moment (will check later when im out of the office).
"What protocol is used in Whatsapp app? SSL socket to the WhatsApp server pools. All messages are queued on the server until the client reconnects to retrieve the messages. The successful retrieval of a message is sent back to the whatsapp server which forwards this status back to the original sender (which will see that as a "checkmark" icon next to the message). Messages are wiped from the server memory as soon as the client has accepted the message"
[1] http://www.quora.com/What-is-WhatsApps-server-architecture