We've added three documents to our repository here on Google Code, each having to do with the Search Appliance. All three can be found off the APIs list. These documents specify:

Appliance Search Protocol: This allows customers to work with the search results from the appliance.
Feeds Protocol: This allows customers to write a custom connector to feed data into the appliance.
Authorization Protocol: This allows a customer web service to authorize users to access specific documents for searching in real-time, leveraging their existing security and access control environment.

These were previously accessible to appliance customers but we wanted to move them onto Google Code.