Ouch :-)

Myths and realities with Hyper-V (external link)

The best bit is

"With Windows Server 2008 failover clustering, an administrator must
store each virtual machine on an individual LUN. Because an
administrator must provide all cluster nodes with access to the
same shared storage by using the same drive letters, 23 is the
maximum number of virtual machines that can run in a failover
cluster. Microsoft IT could work around this limitation by using
mount points and virtual machine groupings, but it considers this
configuration too complex to administer. Because of this
limitation, Microsoft IT has adopted a standard of using only three
nodes in a cluster, with the cluster configured to tolerate one
node's failure."