Exchange 2007 has finally been released. It is available to Volume Licensing customers on licensing.microsoft.com.
There are no bits on MSDN or Technet yet (will be there soon) but you can get the evaluation copies already and remember that Microsoft is now distributing evaluations as VHDs as well as ISO’s. So if you want to just play with a fully configured and provisioned Exchange 2007 environment right away you can bypass the process of installing Windows 2003, then installing and configuring Exchange, etc, etc. It’s like your local personal Technet virtual lab.
Again, congratulations Microsoft.