I've encountered a fair amount of confusion around Hyper-V and what you need to obtain Hyper-V.
Hyper-V is available as a Role inside Server 2008, however it is ALSO available as a stand alone Hyper Visor platform without a surrounding Host OS GUI.
Have a look at http://www.microsoft.com/servers/hyper-v-server/how-to-get.mspx for download locations and comparisons between the different editions of Hyper-V as a role built into Server 2008 and Hyper-V Server 2008 as a stand alone FREE product.
Nope that you wont find information on Hyper-V 2.0 here yet, since Hyper-V 2.0 is part and parcel of the Windows Server 2008 R2 Beta. For More information on Hyper-V 2.0 please visit http://www.microsoft.com/windowsserver2008/en/us/R2.aspx and http://www.microsoft.com/windowsserver2008/en/us/R2-virtualization.aspx
How does Exchange fit into that? Here's a really good place to get started: Microsoft Support Policies and Recommendations for Exchange Servers in Hardware Virtualization Environments which discusses the realities of sizing your parent and guest OS.