Comment 19 for bug 1033226

An update never "forces" you to restart the OS. But when there is a security update for a component that Ubuntu does not have the technology to restart in-place (the kernel, glibc, etc), for as long as you don't restart, you are at just as much risk as if you hadn't installed the security update at all. (This is why Oracle sells Ksplice, for example.) So it's just as important to get you to restart as it was to prompt you about the update to begin with. That updates requiring restart aren't flagged ahead of time is bug 255443, and bug 802065 suggests a way of making the restarts quicker.