Well now; it appears Bungie have given some more information. And its as juicy as the best fruit!
The Skybubble!


Amazed?
Me to

Appears this 'skybubble' is above Sandbox, pretty high; like the roof of the map is VERY high. And the Grid is a physical floor for all the items, but for players; it dosen't exist and players as a result; fall to their death.
Anyway; heres the link to the Interview:
Link
And heres a link to all the screenshots: including some of the Skybubble, and creations:
Link
Do enjoy
