Warning: long post ahead.

Are there any plans to let packages specify that they do not require a total system reboot to be updated? 

The other day, Gnome software prompted me to reboot just to update google chrome. Given that nothing depends on chrome, and also that the Linux version of chrome is specifically designed to tolerate having its files on disk overwritten (http://neugierig.org/software/chromium/notes/2011/08/zygote.html), rebooting the whole system seems overkill to ensure a successful update. 

Presenting users with an option to restart only the affected processes and not their whole computer would improve security by reducing the friction when applying software updates. While restarting a browser is no big deal because it takes no more than a few seconds to complete and all your tabs are restored, restarting the computer is far more disruptive. The latter procedure not only takes much longer but also closes all the other windows the user has open, and the state of those windows is typically not preserved across reboots. The average users would likely put off updating their web browser if they were asked to quit everything they are doing.

On OS X and Windows, user-facing apps typically manage their own updates, and the well-designed apps make the experience quite painless. For example, both Firefox and Chrome on OS X download new versions in the background and prompt the user to relaunch the browser once the update is fully prepared. The user transitions to the new version seamlessly.

On Ubuntu, Firefox is updated by the system package manager but prompts the user to relaunch the program if it is running. This is probably ubuntu-specific behavior and I don't know how it is accomplished or how robust it is, but that would seem a nice example to follow, at least in terms of UX, if it could be implemented using general methods.

Just a stupid idea: perhaps one can specify some dbus method that 
that applications declaring support for online updates could implement.  Prior to initiating the update process, Gnome software could 1) check to see if all the applications in the queue have declared support for online updates (say, in their appdata manifest), and 2) call said method to warn each app that it is about to be updated. Each app would then decide how to proceed; for instance, it could do nothing, ask the user to save and quit temporarily, or trigger some session-saving mechanism.

In the long term, this issue will undoubtedly be addressed as part of the push toward frameworks and a clearly defined "base platform". In that future, the software updater could prompt the user to reboot for framework updates while giving applications the option to handle some aspects of their update process.

But is there anything planned for the interim? Prompting users to restart the system to update a browser seems less than, and the last thing we want to do is condition users to postpone updates indefinitely like they grew accustomed to doing on Windows.