Microsoft has decided to open the public betas of Windows Vista, Office 12, and Exchange Server 12 up early by allowing people to sign up in advance for a spot and be notified when the beta is available. The TechNet email system will enable you to be one of the first to enjoy the public beta. From the Microsoft site:

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 More >