For anyone who's curious, I found the answer with a little more searching:
Even though this Help document addresses a completely different issue, changing the value of AsynchronousSave to false prevents the screen whiteout when saving a PNG. Apparently the default is already false on Mac OS X, so you should only have to change this if you're running Windows (as I am).
The Help document states, "Editing a document during a save operation causes Fireworks to stop responding," but regardless of the value of AsynchronousSave in the Preferences file, you can't edit a document while it's being saved anyway. So I have no idea what the intended utility of the screen whiteout "feature" is supposed to be.
Also, on my system, the path for the Preferences file was different than what was indicated in the Help document:
C:\Documents and Settings\[my Windows login]\Application Data\Adobe\Fireworks CS5\en_GB\Fireworks CS5 Preferences.txt
Two case scenerios this comes in handy (that I can think of):
1. multiple document scenerios, particularly larger files that 'may' take awhile to save. the feature allows you to toggle/work on another document while the other is 'saving'.
2. in single document cases, ever sit next to a person at a computer is is 'click happy'? They click 'away' before the 'triggered' action is complete and freak out why the screen has gone amuck ('non-responding, out of memory, freezes etc). This happens in any program. The feature acts as a 'preventive' measure to ensure the triggered 'save' completes without the end user attempting another forced action.
Valid points. I understand that the application needs to lock the document while it saves. I just don't think the whiteout effect is necessary and, subjectively, it was causing me some undue eyestrain. I've used Fireworks since Version 3 (though I skipped several CS versions, ha) and normally it just shows an hourglass icon when it saves a large document. Fortunately the method I outlined disables the whiteout effect and restores the familiar (and less bothersome) hourglass icon. Didn't think I'd ever be glad to see that icon (since it normally means, oh great, I have to wait).
Good find - would be ideal to have that (any many other in the preference file)
available as user controlled preferences instead of tucked away in