Comment 18 for bug 494772

My 2 cents:

I suppose we are talking about defaults. Provided the configuration options aren't removed...
We want to be on the safe side, because:
- tech-savvy users are responsible for their (informed) choices (unless we induce them in error by way of bad phrasing / erroneous info / etc ), they may change the default behaviour if that isn't made too difficult.
- regular users who need to do some (light) administrative tasks rely on the system to help them.

So, if we really consider a good practice (or a necessary evil) to open a window out of nowhere in front of whatever the user is doing, because he must know that crucial updates should be applied, that window should explain in clear wording why it is so, and that's all.
It isn't very plausible that an ordinary user will want to pick which updates to install...
If, on the other hand, I consider myself an advanced user, I will dismiss that window and open up update-manager / synaptic / aptitude / terminal, and take proper action.

I would sugest one of two approaches:
1. Continue to use the automatic, behind the scene check and prompt for install, but use a simplified update manager window:

"There are critical updates available for your system. It is highly recommended that at least the security-related updates be applied. This may take some time (insert a reasonable estimate?) and should not be interrupted.
[Please note that you are now running on battery, with xx:xx estimated time remaining.]
You can learn more about updating your system in the documentation section [link to proper documentation section, so the users won't stay dumb forever]
(Show me more info) (Install later, when on AC power) (Install the security updates now)"

2. Just don't do update checks while on battery (and also perhaps while using mobile broadband, or... - there are plenty of reasons for not doing that automatically...), but put up an icon in the notification area, along with a notification, to alert that the system would like to check for updates. Periodically, we might even put up a dialog:

"It is advisable to periodically check for updates to your software. This check will happen automatically when you are connected to the Internet, but is disabled when running on battery.
The last time you checked was ... days ago. Do you want to check now?"