I've noticed today (with Activity Monitor running with a memory usage dock icon) that no matter how many programs are running at the same time, the same amount of memory is active (used) and same amount is inactive (used for cache). Only when I've closed a program does the active amount drop and free memory increase considerably. After a few minutes, the memory usage recovers. I'm very impressed with this and never seen such good memory usage before.
So, htmlmasterdave, I wouldn't worry about it 😛