Since recently, my MacBook running ML seems to forget the resolution of my external monitor after awaking from standby.
When I detach it and attach it again when the MacBook is running, everything works great.
However, when I attach the monitor when the MacBook is in standby (e.g. when I take it out of my bag), the external monitor is set to "Scaled - 800x600" instead of the native resolution of "Best for display".
Resetting NVRAM didn't help.