1) When it shows you the upgrade dialog, it quite clearly notes if you’re going to need a reboot or not — both on the short view (“You need to install 1 update”) and the list view of updates. The list view even tells you which updates require a reboot, and which do not.
2) After upgrading, it runs itself again to check for more updates. Vista/XP desparately need to do this (it seems like I always have to run it 2 or 3 times to get all the updates). Strangely despite implementing this awesome feature, the Mac never seems to need 2 or 3 waves of updates.
(The main reason this is nice is it popped up while I was remoted in. I didn’t want to have to reboot since VineVNC server is somewhat crappy and doesn’t consistently start up until I’ve logged in).