It is OK to VM the SQL Server instances but I wouldn't vm the data with it. The nodes need to see the same disks (as Steve has said).

Still VM is some order slower than a dedicated box, as everything is being shared from network connections to CPU cycles. Some intelligent analysis and testing would be in order. I wouldn't use it for HA systems unless those systems are strictly lookup systems with DBs that can reside in memory, or are extremely focused and minimal in overhead.