I caught this post up on my colleague's blog on Hyper-V and support for Exchange. http://blogs.technet.com/jkruse/archive/2008/06/27/hyper-v-is-here-what-about-exchange.aspx

For those hosters who are considering delivering a Hyper-V based solution for virtualised Exchange solutions please see the following detail:

In case you haven't heard the news, we have released Hyper-V into the wild.  In fact did you know that 25% of microsoft.com is already running on Hyper-V VMs?

So what about Exchange Server?

As we recently announced at Tech.Ed USA, Microsoft will be publishing a support statement with respect to Exchange Server on Hyper-V within 60 days of today.

Some of the more noteworthy bits of info from the session are:

• Within 60 days of Hyper-V's RTM, the Exchange team will publish a detailed support statement for Hyper-V, and a TechNet article with best practices.
• Customers should not deploy Exchange on Hyper-V until our support guidance is available.
• Early results show that performance of Exchange 2007 on Hyper-V is quite good, and that it scales very well from 1-4 processors per VM.

In terms of best practices and requirements, the following guidance was released:

• We're only supporting guests running Exchange Server 2007 SP1 on Windows Server 2008.
• We're only supporting fixed disks, and not dynamic disks.
• Storage should be on spindles that are separate from the Guest operating system VHD physical storage.
• Storage must be SCSI passthrough or iSCSI (with a preference of SCSI passthrough for queues, databases and log files).
• All Exchange server roles, except for the Unified Messaging server role, will be supported.
• There's a limit on VHD size of 2040 GB.

Let's not forget however that the recommended method to build a hosted Exchange platform is to use the HMC 4.5 guidance found here: www.microsoft.com/hosting