I've attached a blend file that shows the problem. I originally created it in 2.49 and it had the same memory leak there. I just tested again in 2.5 (self built from svn on Ubuntu 8.10 AMD64) and it still has the same problem.

Each time the scene is rendered, there is a significant memory leak shown in the (memory map) value. As the scene is animated, this quickly leads to massive swapping and extreme slow down until the system becomes non responsive

Disabling Compositing frees the memory map and it doesn't show the 100mb+ increase on each render while it's disabled, so I suspect the problem lies in freeing the temporary data created in the node network.

I tried deleting parts of the compositing node network to isolate the problem, but it doesn't seem specific to a single node as the amount of memory lost changed with each deletion.

Safest way to test is just to press F12 a few times and watch the memory map value after each render. Rendering the animation will end up needing a reboot if you let it run for too many frames.