When do the armed speed brakes of a B737-800 extend after landing?
Is it when the main gear makes contact with the ground, or must the nose gear also be on the ground?
The system will extend the flight spoilers when any strut (either main gear or the nosewheel) compresses; the ground spoilers deploy after the right main gear strut compresses. Everything will deploy even if the nosewheel is held off the runway, although that's not a recommended landing technique.
Additionally, other conditions are required:
Additionally, the system will deploy the spoilers without detecting strut compression if it senses main gear wheels spinning up to 60+ knots (presumably both sides, but the manual I'm looking at isn't clear on that).
Source: an airline manual that isn't public domain, so no link or screenshot - sorry.