I've pasted the following within my prior reply:
"PS3 SDK 1.80, OS memory reductions, new features
OS reserved memory: 48 MB (main) + 24 MB(vram)"
But reply from Heavenly Sword dev (august 2007 at Beyond3D forum): "1.80 was out absolutely ages ago.. 1.90 has been in developer's hands for a few weeks."
So I don't know the current & future figures, thus I cannot really comment to getter depth. One thing is clear, they are making improvements. It can be they are reserving some memory for future still disabled (under development) features as well.