Register for public beta releases
You can now go to the TechNet Beta Central site and pre-register for the upcoming public beta releases of several products, where among others you can find Exchange 12.
"Lead evaluations for new technologies and understand their impact on your IT environment and their potential for your company by pre-registering here. You will be the first to be notified when the Windows Vista, Office "12," and Exchange "12" public beta releases become available. Additionally, you will receive updates and information regularly through the Microsoft TechNet Flash newsletter, from the time you pre-register until the launch of the product."