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 have a 23" iMac running Lion (10.7.3). I've checked that the "Enable HiDPI display modes" checkbox is enabled in "Quartz Debug" (it was checked the first time I opened it), but there are no HiDPI ...