The original frustrating issue: my other half’s 13″ retina Macbook Pro was unable to put the external display into sleep. Video signal was off, but something kept the display always on (it’s a Samsung SyncMaster 213T – which isn’t a young one, but still a very capable monitor). So I tried a crazy idea: swapped my Mini Displayport to DVI adapter with hers.
And the result was – to my biggest surprise – proper display sleep. I was curious whether it is a faulty adapter or what, so tried the non-sleeping adapter with my EIZO CG241W. The result? Proper sleep behavior again.
The only difference between the two MiniDP to DVI adapters is that mine is almost three years old, while hers is from last December. They look exactly the same, have the exact same part number, etc.
Conclusion: if you encounter display sleep issues, the culprit might be compatibility between your adapter and monitor. Try to get an older Apple adapter or try an aftermarket one.