I have seen that before but ..have you tgot he latest update installed?
I have the same problem, only it keeps 100MBs of memory.
It's been like that since version 5.0 through 5.0.1 and now 5.0.2.
Adobe Media Encoder does the same thing for me, also that 100MBs and from version 5.0 to 5.0.1.
None of them use any CPU while idle in memory for me though. It looks like the same issue for both programs...?
I had the same problem few months ago and solved it by updating NVIDIA graphic drivers.
Hope this helps.
I have the same problem with most of the CS5 programs, and it is new with CS5. It doesn't really effect anything except that I constantly switch between Premiere and AE. I render something in AE, place on the timeline in Premiere, switch back to AE and make changes, re-rerender, etc. The problem is that CS5 has another new bug that prevents you from being able to re-render an AVI file that is open in Premiere. Instead of the usual behavior of Premiere updating the AVI file on the timeline after it has been re-rendered in AE, AE gives you a busy file error and will not render the file until Premiere is closed.