It appears that the Dev Center at Apple just updated its documentation set today. If you're using Xcode 3.2 and you want to update your documentation, you might be looking in the wrong place.
Before 3.2, you used to update your documentation in the Developer Documentation window (Help > Developer Documentation, or Command-Option-?). Now you'll find your documentation sets in the Xcode Preferences window (Xcode > Preferences... or Command-, and then choose the Documentation tab). Also, if checked, Xcode will automatically update your documentation when you launch it.
This Documentation panel offers subscription options for installing a documentation set (such as, for example, Mac OS X Legacy Library or iPhone OS 2.2 Library) as well as a handy Check and Install Now button that lets you request the latest updates. Use this to keep on top of the latest documentation updates.
Thanks, Scott Lawrence and @zadr