Exchange 12 Beta 1 CTP now Available for download on MSDN
Basically, it's the same as the one released to testers. At this stage it is not feature complete. Technet subscribers will also get it as part of their mail subscription.
This the information as posted on MSDN:
Exchange "12" Community Technology Preview (English)
Date/Time Posted: 2006-03-01 22:57:23 (UTC)
File: en_exchange_server_12_ctp.iso ISO-9660 DVD Image
File Size : 1617 MB
To subscribe to Technet and get the CTP: