I have an iMac(27" from 2010) and while running Lion I tried to HiDPI modes that gives me 1280x720 and larger text. I did change back to 2560x1440 long before upgrading my system to Mountain Lion.
If I use the screen saver, or switch to the "select user to login" main screen, and then back to my user account, my windows are resized to fit inside a rectangle that looks like 1280x720. This behavior happened in Lion too and the upgrade didn't change that.
Any idea on how I can get rid of this behavior? I haven't found any .plist file to delete that seemed relevant.