There is plenty of research evidence documenting the business benefits from high-trust business relationships. Unfortunately at times like this (2009), the knee-jerk crisis reaction of many companies is to squeeze suppliers harder rather than to combine a call for greater efficiency with a collaborative approach to finding sustainable solutions.
One study documented in A Guide to Trust (by the Relationships Foundation with Ci) examined supply chain relationships in the motor industry and found dramatic benefits for those with high-trust relationships: Design and distribution ideas were shared, and purchasers handled twice the value of goods when dealing within highly trusted manufacturers. In another industry, Jim Sierck of Xerox USA estimated that the bureaucratic structure created to handle the lack of trust in their buyer/supplier relationships cost them around 7 cents in the dollar.
The evidence is clear: If you want efficiency, you need to release all the available talent, innovation and energy in your supply network. That comes from a focus on building trust, not just reducing cost.

 This download summarises "A Guide to Trust" on a single page. The full report is available here.