Starting with OS X 10.5 there are evident memory management problems in MAC OS X. The web was already then cluttered with complaints about system slowing down dramatically after some time. Back then i had slower machine, Mac Mini with 1GB RAM, so i (wrongly) concluded that it was due to inferior hardware.
Now i have 2010 MBP, core i7, 8 GB RAM, dual GPU.
Mac os X Snow Leopard was pain, but after migrating to OS X Lion, working some serious stuff on MAC started to be a nightmare.
I finally managed to reproduce the problematic scenario, so i run the test and recorded the screen, into video.
I run the tar+bzip command, which is basic unix stuff, on the large amount of picture files, in my Pictures/ folder. Just before start, i run the "purge" command, to delete inactive/cached program data.
You can see on the video that free memory starts to drop very fast, and inactive is constantly rising. If you take a look at "bsdtar" command, it takes only a fragment of RAM, so the problem is not in this process. You cannot say that it is a program memory leak, because then the problem would not be in inactive ram, rather in active/wired.
When the free memory dropped below 100mb, i started some apps, like Safari, iPhoto and MS Word, and you can see in the video, that it takes even minutes to start an app, when normally (when there is free RAM), it would take some 3-5 secs to load.
I run the same scenario and the same commands on my Linux Centos 6 box, no problem there ! Memory usage is some 10-20mb, no problems with cache/buffer.
The memory management must be very broken in Mac OS X !