Why would they allow you to install the game and play without the disc if they don't plan on making the disc unusable afterwards?
What would you do after installing the game? Naturally you would sell the disc for profit right, simply because you don't need it?
So it must mean 2 things,
1) MS is aggressively allowing used games sales on the 720 which devs and pubs won't go for. Because everybody in their right minds are going to instantly resell the games after they install.
Or
2) MS wont allow that disc to be used on any other 720 after initial install, in an effort to block massive second hand sales. Always connected will allow the system to check any disc inserted into the disc bay, if the disc was already registered on another 720, well guess what, some notice will pop up telling you to unlock this game for $60. Pubs and devs rejoice.
Also their is no way MS is going to sing this from the top of the mountain because they know it is a system killer. This freature won't be reveled until the system is in your house connected to our televisions, a few folks will warn of it, but MS will bind most media outlets to nda to keep this from being uttered until launch.
I expect Sony to pull the same shit.