Microsoft Exchange Server 2010 is currently a beta product and is expected to become available for purchase in the second half of 2009.
Until then, you can purchase the existing Exchange Server 2007 product. It can be purchased stand-alone, as part of the Core CAL Suite, or as part of the Enterprise CAL Suite.
If purchased with Software Assurance, you will have rights to the future versions of Exchange, including Exchange Server 2010.
More details about licensing and pricing will be communicated at a later date.