I just tried it out with the SysPref pane open as I couldn't tell what was happening. At least on my system (10.3.9) holding down option changes both the alert volume and the system volume. So if you use this hint to adjust your alert volume you then have to re-adjust your system volume by repeating the exercise without holding down option.

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Just tried it on 10.4.6 and it works as advertised. The levels remain separate. If I opt-click and set alert to like 70%, then click, my system volume remains at it's 40% level. I can toggle back and forth (using opt), and they stay independent. Note, this is w/o the sys pref pane open.

Nice hint - thanks!

PBG4 1.67GHz 17"

Confirmed. Works as advertised here too (10.4.6).

Maybe it was a Tiger implementation (contrary to Rob's footnote)?