The edge of the world was broken in the first snapshots, but we reported it and it was fixed.
However, all blocks beyond +/- 30million from origin now act as solid blocks to the player, so you can't go outside it. Even if you use an editor on your location, you'll be stuck because all blocks around you act solid, including air.
The interesting part is the camera isn't restricted by this. It can pass through the ground if you F5 twice and look down at the ground beyond the boundary. This means it still generates ghost chunks as it has since Beta 1.8, so generation wasn't changed as much as suggested. Just tweaked a lot.