Vizioncore is working on a product called esxReplicator which is capable of doing VMDK level replication across the network. There is a rules engine that can synch your data based on 2 criteria...the amount of time passed (every hour), or after a certain amount of change (32MB). As soon as the threshold is hit, the difference is sent over the wire to a remote location. Because you have full control over size or time incriments, it is a very fast replication once the initial sync is done. Because the differentials are so small, there is no performance impact to the guests in the configuration. I have no ETA on release or timeframes of the product, it was something that they introduced and demod at the VMUG (VMware User Groug) meeting.