Consider the following: IBM's SAN Volume Controller (SVC) is the most popular storage virtualization appliance out there, and now DataCore's SANsymphony is becoming more enterprise-ready to give it a run. (See IBM's Got Virtual Vision.)
These products -- as well as FalconStor's IPStor -- run on servers that sit in the network, or in-band. That's a different tack than EMC Invista, Fujitsu Eternus, Incipient Network Storage Platform (NSP), and LSI Logic Storage Virtualization Manager (SVM) take. (See Virtualization Buyers Keep Exit Open and A Baby Step for Storage Virtualization.) Those products sit outside the data path and work in conjunction with intelligent switches.
The network-based guys, particularly IBM, got out first but had a problem scaling because a second device added to the network didn't see the same pool as the first box. That made them limited as enterprise plays.
DataCore is making a run at solving that problem with the latest version of SANsymphony, which has features such as N+1 availability, support for multi-path I/O drivers that allow use of Fibre Channel and iSCSI paths for failover, and thin provisioning.