I'm running WinME with ACPI enabled in the bios as well as in the O/S. Though I am not running a specific "halt-on-idle" program I think the ones I have tried left an artifact file behind because I see my cpu behave like its running a "halt-on-idle" routine. When I boot up to the point of the dialog window waiting for my password, my cpu temp sits around 15-16C above ambient (1G Tbird @ 10x150) as soon as I enter my password, a few house keeping routines execute and then my temp suddenly drops to about 4-5C above ambient. I'm not complaining though. FWIW, I have dual boot capability and my other O/S is Win98se. Having tried a few software coolers on it also, the same phonemenon can be observed. Interestingly, this does not happen when I pull the Tbird and put my Duron in. Temps are measured using a thermocouple resting on top of my core, through a 1mm hole in the HS base.
So, idle can vary, depending upon what you are running for a cpu as well as the O/S and bios settings.