This is a catchy title, but if you are interested in playing with Hyper-V 3 then add the following to your virtual machine vmx file:
vhv.enable = "TRUE"
hypervisor.cpuid.v0 = "FALSE"
mce.enable = "TRUE"
Once you have added these lines you are good to install the Beta and add the Hyper-V 3 Role. If you miss out the 'mce.enable' line, when you reboot after adding the Hyper-V role you get a blue screen with an error complaining about hal_initialization_failed, so make sure you include it.