Jose Barreto just posted an article describing different ways to implement Windows Server Failover Clustering with Windows Server 2008 Hyper-V.
The scenarios described include:
1 – Parent-based Failover Clustering with two physical servers
2 – Child-based Failover Clustering with two physical servers
3 – Mixed Physical/Virtual Failover Clustering
4 – Failover Clustering with two child partitions on one physical server
5 – Standalone demo laptop with Virtual iSCSI SAN
Check the details on each one (including a “before” and “after” diagram for each) at