Edit 4/20/2010: Replaced the post body with the link explaining how to get most current information on the subject:

Learn about Exchange 2010 Virus Scanning (VS) API 2.6.1 and VSAPI TAP

The VSAPI 2.6.1 provides programmatic interfaces for a Virus Scanning application to access items and attachments stored in Exchange. The VSAPI 2.6.1  is only provided to organizations enrolled in the VSAPI TAP program.