When I fired up Xonotic yesterday, a little yellow bubble started floating around telling me to update.  The game was still playable, but it was enough of a distraction that I will update pretty soon.
Could anyone outline a bare-bones way to add this to vanilla Quake?  Something that nags a little but leaves it to the user to download.  I imagine something like checking "http://mygame.com/version.txt" for a match.  If it doesn't, centerprint the content of  "http://mygame.com/upgrade_message.txt" every few minutes.