There's ZREP [1]. It uses the snapshots feature of ZFS, which can send them over the network, to continually (but asynchronously) mirror the local filesystem to a remote machine.

It seems to provide exactly the features required (fast local access with failover), at the expense of a window of lost data (since the replication isn't synchronous and confirmed).

[1] http://www.bolthole.com/solaris/zrep/