Re: dont hate me for this question
Well, here on Fedora I use the livna pre-packaged RPMs and I never had any problems installing or upgrading them directly in X11 (even using the GUI package installer).
Maybe it would make sense to propose the user a reboot after installing the drivers from X11 (and perhaps issuing a warning before starting installation with X11 active), but I don't think forcing the user to install from the text console (many newbie Ubuntu users don't even know about it) is a good idea.
Or alternatively, maybe it makes sense for the nvidia installer to shut down X11 itself (after asking the user about it, of course) and doing everything in text mode without user intervention.
Anyway, it's just a usability question.