Comment 14 for bug 73244

win32 doesn't delete the file and the helper applications are holding a file lock open on this files. I'm unsure about OS X.
Leaving them in temp is no alternative solution for every os that doesn't delete the temp files. Some users will end with gigabyte of wasted space.

It might be the best solution on Debian/ubuntu but what about Suse or Fedora ?
Deleting on every reboot can be something that is not fast enough if you never reboot your linux system and the user downloads several iso files every day and opens them with a DVD-Burn application. (worst case scenario)

Maybe watch if the helper application process is gone and delete it after that ?