It might be based on a time limit, i.e when the clock says Aug 1st. It wouldn't surprise me if there is also a patch sent via updates that will stop the OS from working (to stop those that have the clock set in the past).
Then that is easy fixed before the cut of date you disable updates and then set your clock in the past..
Do you think it will be easy to update from the beta to the windows 7 when it comes up.. like installing it or reformating the whole OS again?