ZFS support is now officially there. What would have been impossible through conventional business relationships has been achieved through open source.
Nexenta Storage Appliance is designed and built to operate as 2nd tier storage alongside pre-existing commercial storage, providing online continuation of data for months and years, with tapes relegated to archival purposes only.
An Ask Slashdot post dealing with the issues surrounding a client/server synchronization with a laptop. The question focuses on being able to automatically propagate writes to a server when connected to the network, but use local data when disconnected.