For a long time, Apple's put an LED on all Macs. When it's flashing, the machine is asleep. Modulo some limitations on "deep sleep" imposed by non-compliant PCI cards, sleep has been very simple. When a Mac is asleep, it's almost off -- it consumes very little power, and you can't do much besides wake it up. When you wake a Mac up, it comes back to full functioning quickly.

I have a MacBook Pro at work, and strongly believe computers should never sleep -- I do most of my job via ssh, without looking at whatever system I'm working on. So I always set System Preferences > Energy Saver > Power Adapter to Never sleep.

Intel-based PCs have several different low-power modes, including at least one ("hibernate") where they save the contents of RAM to disk, so the machine can completely shut off but still "wake up" without booting again. This type of behavior will feature more prominently in Vista. Apparently booting Vista is so slow that people are going to spend real money for large flash drives to reduce booting...

Anyway, Apple has apparently redefined the meaning of the white LED on the MacBook Pro. Even though I'd set cayenne (my MBP) to never sleep when plugged in, I kept noticing that while on my desk, the screen would go black and the LED would flash. "Hmm, this is not as intended!", I thought.

I checked "pmset -g", which confirmed sleep cayenne was configured never to sleep when plugged in, but it kept happening. I called Apple to ask about this, and was told that the LED meant the MBP was indeed sleeping despite the configuration. I reset the power manager a few times, and Apple sent me a box. Wednesday I got cayenne back, with an upgraded logic board, but after sitting on my desk, the screen again went black, and the "sleep" LED again started flashing.

"Hmm," thought I, "perhaps Alex was right and 3 Apple Support reps were wrong." Alex had told me that the LED flashing on his MacBook did not mean it was in fact sleeping, but foolishly I believed Apple Support and my own historical experience instead.

I was able to ssh into cayenne while the LED kept blinking, which proved that it wasn't really asleep in the PowerBook sense at all.

Moral of the story: Apple upgraded my logic board through ignorance of its own equipment, and I lost use of the MacBook for a week because I didn't take Alex's word for it.

Update: It's more complicated. The documentation doesn't match my experience, and Apple actually started changing the sleep behavior with the next (final?) generation of PowerBooks, released immediately after my 1.5GHz PBG4.