Sometimes, I would want to install something while either an update or another installation is in progress.
Apt-get doesn't allow more than one installation at a time, and I understand that. But it can have a queue, so packages would be saved in a queue, and when the updates or first set of packages finish installing, the second set of packages in the queue would commence.
Fortunately, PackageKit will solve this problem (among another ones). APT backend for PackageKit is almost complete and Canonical has showed an interest in PackageKit so probably this feature will be implemented quite soon (maybe Intrepid+1 or +2).
Just look at fedora. It has a queuing of update/install tasks. That's quite useful. But fedora uses packagekit. And perhaps this "problem" will be solved in Jaunty Jackalope.