A beta of Exchange Server "12" has been available since December, but only to a limited audience (internally at Microsoft and to a group of customer participating in the TAP - Technology Adoption Program).
As of March 1st, however, a Beta 1 Cummunity Technology Preview (CTP) has been made available to a wider audience, via Technet and MSDN subscriber downloads. See details on how to subscribe in the sites below.
The next step is the Beta 2, which should be available in mid-2006. The final release is expected in late 2006 or early 2007. As usual, the best place to keep up to date with the latest Exchange developments it the Exchange team blog at