When the Beta expires all that will happen is (as stated above) nagging warnings will start popping up about how "your copy of windows is not activated"... perhaps some limited functionality... things like that. Your computer isn't going to suddenly "break" or stop working alltogether. Ya just have to remember that it's just a BETA. meaning it shouldn't be used as a substitute for a full versioned OS....
Especially if your not familiar with Beta software.. And please remember to back up your personal files before it expires on the off chance that something happens to them. (It's better to be safe than sorry so they say... ) As far as the RC and RTM being open to the general public.. unfortunately it's not going to happen... I really...