If I understand correctly you running Mac OS X with dual displays. At work we have the same problem with OS X and the secondary monitor, Its a planar lcd in our instance. I searched google and many Mac users are having this issue with the secondary monitor not regaining a signal after coming out of sleep mode. The only solution I know of is to turn off sleep mode.
Yes, I am running OS X but not on a real Macintosh, I build high quality Intel-based "hackintoshes" instead. However, I did not have this problem with the Samsung panels I just replaced, so I am pretty sure this issue is related to this particular HP ZR24w which is otherwise flawless.
I also want to note that in this dual display setup that I believe the panel in question is the primary one both in terms of OS X (it has the Main Menu bar) and also in terms of the hardware (the bottom DVI port is connected to the panel in question and I believe that is primary on the EVGA 9800GT card I am using).
Since this rarely happens (twice in about a week) and the fix is so trivial (hit the power button off then on), it is no where near the point that I need to be concerned about finding any kind of real solution or even consider disabling sleep. I just want to document the issue in case it turns into something significant or it is found to be related to a more general defect of the ZR24w (e.g. last week someone was complaining that their OSD settings were lost on sleep).
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