I recently was working on a project on my MBP using battery power, and had just bought a new battery so I was going to try hot-swapping the new battery in during sleep mode. I closed the lid and waited for the Safe Sleep image to be created, then, when the light started "breathing" to show sleep mode, I pulled out the old battery. The sleep light went off, and after putting in the new battery, the MBP woke up using safe sleep.
I was given the impression that the MBP had an internal "backup" battery for this purpose, to allow hot-swapping without having to restore with Safe Sleep. Have they removed this feature? I'm using the June 5, 2007 revision 15" MBP.