By default, a Mac laptop will write memory out to the disk before sleep mode.
It's only supposed to do this for zero power sleep (what's called hibernation in Windows world). That's supposed to be triggered automatically when the battery level is very low. Have things changed in the past year so that this is happening all the time?
My initial Intel MacBook Pro wrote out the memory to disk every time, as does my current system. From what I understand, this is normal. The machine still goes into normal sleep, with RAM contents being held by using battery power. If the battery drains completely, then it will resume using the on disk memory image, aka resuming from hibernation.
The reason it was implemented this way was to support hot swapping batteries. The older Powerbooks had a separate internal reserve battery used for keeping things alive long enough for a swapout to occur. The Macbooks got rid of that in favor of the hybrid sleep/hibernation support. My guess is that they also didn't want to wake a system near low power to write out memory, as it may be in motion at that point, triggering the hard drive to remain parked.
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