PC clustering used to be a very specialized discipline. You needed to set up nearly identical computers and run special operating system versions to keep them synchronized. They required special networking adapters to move data between the two PCs at very high speeds. But networks have gotten faster, so that gigabit or better Ethernet is now found on most desktops and servers. And a combination of services, including high availability, virtual storage management, and near-term server failover that were previously only the province of very expensive and customized clustered configurations, are now available in the virtual world and can serve as a good substitute for many enterprise's disaster recovery (DR) applications, too.
Certainly, virtualization is on the rise for many enterprises: a combination of better resource use, reduced power and cooling in the data center, and more manageable application delivery have made it a very popular solution recently. And as IT shops gain more expertise in delivering virtualized applications, they have seen that they can also get a better handle on how to leverage this expertise to deliver clustering solutions.