Technically, we've enabled offline synchronization between devices, but our approach is littered with caveats:
Offline syncing only works over Wi-Fi
Requires a laptop to perform synchronization
Involves a manual setup of a separate CouchDB database, a peer-to-peer Wi-Fi connection, and static IP addresses
Cannot sync two mobile devices together (unless syncing through a laptop)
Have to edit the source code to switch to offline mode
A lot of these reasons are due to technical constraints. Let's go through each point and understand what the ideal solution would be, whether possible today or not. There's a huge opportunity for mobile device manufacturers to improve the current state of offline synchronization in a platform-independent way.