Currently Java updates are delivered based on the existing JRE version on the target machine. And it's the same irrespective of the target OS.  It would be nice to be 
able to control the updates per OS: that is some new updates can be targetd only
for specific OS's: Windows Vista, Windows XP, Windows 2000 etc.
On top of diferenciating based on os, we should also allow diferenciating based on arch and locale, since we will eventually suport auto update on both "i586" and "n64" or "amd64" platforms, and we may want to provide an auto update only in some locales.
Any other flexibility we can build in now will help us down the line.
The eng spec is at: http://j2se.sfbay.sun.com/web/pub/Deployment/ConsumerRelease/6518205.html