Yes, it's a bug. It broke in CS5. We have a fix in development now.
Sorry, but I can't say right now.
Bummer. So much for using aerender as a background rendered while
working. The box becomes unusable. There goes my team's workflow
If you're patient and you watch the render go along, you can type in "purge" in a separate terminal window to free up some ram. I usually have to do this about every 10 minutes- but it is still worth being able to render multiple comps in the bg.
Hope there is a real fix soon!