I have a Mac Pro running 10.6.5 Mac OS, 16GB Ram that I just installed today (had 8GB). I opened up a rather large 11 minute PPro project. I rendered it out. Noticed that the Used Memory was getting larger, up to about 10GB where it leveled off for the remainder of the render. After the render was done, the used memory stayed at 10GB. I exited PPro, and it stayed 10GB. Seems like it's not releasing the memory back to available or am I not understanding how either the Mac OS works with regards to memory or perhaps PPro?