Are you sure? (you know the video by now, I guess, thus I only mentally link to it)
It does not work for me in Mac OS X 10.6.4, either by holding down the Option/Alt key and dragging the file to another folder on the same volume (just makes a duplicate) or Option/Alt key + drag to another volume, the original file stays in place on my Mac OS X version, thus my assumption and certainly "arrogant" declaration of that fact.
Maybe I should read the threads I linked to, thus I can answer the OP the question s/he asked.
Anyway, it does work for you (Option/Alt + drag & drop)?