Apache processes at 100% with no traffic - how can I see what's going on?
Solution 1:
Found the issue. It was a code problem with Magento. The image resizing function was failing to open an image, because the extension was incorrect (it was a BMP with a jpg extension). The error handler for this was invoking the resize again, et voila - a loop. Found this by doing strace on the misbehaving apache process.