Confirmed anyone who pre ordered can get in at 5pmEDT on 10/16 (today for those of you who have no idea what day it is).
To make that a tad simpler you can play the story now though you have to install it through the "Head Start Access" install and play through that.
I have no idea if you will need to install the full game again once it is released and there is no info on the website that gives any confirmation one way or another.
The website also confirms that your experience and characters from head start five (the current early version) will carry over to the full release. They are NOT confirming that any previous head start access characters will carry over.
Character creation and most of the Dungeon Master tools are now available. Some DM tools are withheld to avoid plot spoilers until full release.
Anyway this is the longest post I have made on this website (most of my other posts are the words "reboot your computer to fix that". So instead of droning on like some kind of fanboy I will instead just link you to the FAQ for the head start: