Mountain Lion display sleep function and screen saver does not activate
After a long while now, the problem has come back.
I found very detailed and useful info at http://web.archive.org/web/20150405031645/http://mactips.dwhoard.com/mactips/system/display-sleep. For me the workaround was just to logout and login again. Then test and the problem is fixed. The referenced page gives other, perhaps better workarounds.
Please note that the normal &quot;why doesn't my Mac sleep&quot; issues such as given at http://support.apple.com/kb/HT1776 are not the problem, and that some bug is the problem.