I just came across the Wiki article on Lauda Air Flight 004, which was destroyed when a thrust reverser activated in mid-air.
In the article, it says:
The incident led Boeing to modify the thrust reverser system to prevent similar occurrences by adding sync-locks, which prevent the thrust reversers from deploying when the main landing gear truck tilt angle is not at the ground position.
But that can't possibly be enough. Landing gear is lowered and locked while the plane is still flying. That means these "sync-locks" would be released while the plane is still in the air.
Edit: Looks like I misunderstood what "not at the ground position" meant. Apparently the truck tilt angle is not flush except when the wheels have pressed onto the ground. Nevertheless, the general question of what safety mechs there are, still stands.
Are there any other safety mechanisms? Are there any that detect, somehow, that the wheels have actually touched down? Maybe a wheel RPM sensor?