A B767-300 returned to the departure airport following a loss of pressurization control.

Narrative

Our day began with the Load Supervisor informing us that the aircraft would be participating in a 'mock deicing' drill after engine start at the ramp throat. 'Deicing crews will be spraying the aircraft with water. The water is dirty so make sure the aircraft is properly secured for deicing procedures because of odor.' Procedures for deicing were followed by the crew. We 'mock deiced' and continued to taxi and take-off uneventfully. Climbing through 11;200 FT we received an EICAS message and aural alert for 'Cabin Altitude' with no other EICAS messages. We performed the immediate action items; descended to 10;000 FT; and slowed. We then performed the 'Cabin Altitude' checklist to its completion. Shortly thereafter we received an EICAS message for 'Cabin Auto Inoperable.' We performed the 'Cabin Auto Inoperable' checklist. We tried to manually control the pressurization. We then coordinated with Flight Control and Maintenance for an Air Turn Back. The decision was made to return to the departure airport. During descent and on the Approach Checklist we performed the RECALL action on the EICAS. We then had an EICAS message for 'Main Cargo Door.' I inquired about declaring an emergency and the Captain declined. We landed uneventfully. After the flight maintenance informed us that they found a faulty proximity switch. At this time it is unknown if the contaminated water sprayed on the aircraft during 'mock deice' had any effect on the switch failure. After landing; in discussion with maintenance; the Captain stated positively he had 3 green cargo door lights and the main cargo door was locked before flight. Maintenance personnel also reported a visual inspection of the door prior to flight confirmed the door was fully closed and locked.