If you extract the web installer and run the setup inside it this file shows up (was an INF but changed it to txt for convenience sake),
dxupdate.txt
It is also not there when you extract but only shows up after running the setup.
From the list of what it is checking on the Microsoft server it almost looks like they haven't updated it on their end yet seeing as the latest files listed in that are from June 2010. As far as I know the web installer doesn't really change from one update to the next, they just change what it looks at when it queries the download server and compares what is already installed.