Apple publish the Xcode docset through RSS feeds, so Xcode can check and download them easily. If you are interested in retrieving the docsets directly, here is the catch:
- The feed for Snow Leopard docset can be viewed at: http://developer.apple.com/rss/com.apple.adc.documentation.AppleSnowLeopard.atom
- Each entry describe a version of the docset. You can notice that the attachement is a xar file. This is the archive that contains the actual docset
- Once downloaded, the archive can be extract by using the following command: xar -xf <archive>.xar.
Of course, this is valid for other docsets.