Anything particular about the files (Color Mode, size, …)?
Are Photoshop and OS fully updated and have you performed the usual trouble-shooting routines (trashing prefs by keeping command-alt-shift/ctrl-alt-shift pressed while starting Photoshop after making sure all customized presets like Actions, Patterns, Brushes etc. have been saved and making a note of the Preferences you’ve changed, 3rd party plug-ins deactivation, system maintenance, cleaning caches, font validation, etc.)?
Ok done all that and with the same result. I'm in very small PSD 8 bit RGB Adobe 1998 document.
Ok I did yet another computer restart, it failed to reboot and sat there with a light blue screen. I hard booted and hey presto my displacement map now works!!!? I think Mountain Lion has a lot to answer for!