Two new documents were released recently that describe the new Hyper-V features in Windows Server 2008 R2 currently in Beta and how to implement them in a test environment. The first document details the much discussed Live Migration feature and the architecture behind it. The second is a step by step guide for setting it up to see for yourself. If you have set up Hyper-V host clustering previously, it is quite easy to set up and utilize the new features in R2. If you haven't previously worked with host clustering, be sure to follow the step by step guide closely and it shouldn't take long before you are up and running.
I've had the beta running in my lab for a couple weeks now. I have a 2-node cluster using the 2008 RTM Hyper-V and a new cluster running the 2008 R2 Beta. So far Live Migration has worked perfectly in my limited testing. The failover is seamless and a continuous ping against a migrating VM has never exceeded one drop. I haven't tested a lot of running applications during failover yet, that is up next.
I'm also a huge fan of Cluster Shared Volumes (CSV) which allow you to store multiple HA VMs on a single LUN. This provides much easier management of storage which is always a good thing.
Windows Server 2008 R2 & Microsoft Hyper-V Server 2008 R2 - Hyper-V Live Migration Overview & Architecture
"One of the most highly anticipated new features in Windows Server® 2008 R2 Hyper-V™ is live migration. This document describes the live migration feature of Windows Server® 2008 R2 Hyper-V™ in detail, including how live migration moves running VMs and requirements for implementing live migration"
Hyper-V: Step-by-Step Guide to Using Live Migration in Windows Server 2008 R2
"This guide details the steps required to perform a live migration of Hyper-V™ virtual machines from one node in a Windows Server® 2008 R2 failover cluster to another node."