How to remove leftover files after an application download? I don't have a lot of space on my phone (an HTC Aria).  When I download an app from the market, I usually move it to SD immediately, but I've noticed that the "Internal space" amount doesn't change for a little while.

I'm assuming that before installing the apk, it keeps a copy of it in a location on the internal drive, then garbage collects or cleans up the space sometime later.  My question is, is there any way to force this cleanup?
You are completely right with your assumption: Whenever an app is installed, it is first copied to /data/local. You can check that directory if either you've got a file manager permitting you to go there, or have the Android SDK installed. No root needed, as this directory is readable and writable for the shell user.

But to your last question: No, I'm not aware of any mechanism to control that delay (never saw a corresponding setting). If it can be configured (and is not hardcoded in some place), this will most likely involve root privileges.