All of our city-wide software is run on large dedicated application servers. We all share the same code and fonts and all I have to do is upgrade in one place and everyone picks up the changes the next time they launch the application.
It took me 1 minute to upgrade OpenOffice from 2.0.2 to 2.0.3 for 700 users. In the shot below, users up to 12:20 launched the old version. Users after that time picked up the new one. Those in the old version are not logged off. The two exist at the same time, and 2.0.2 is retired when the last user logs out tonight.
All done. :)