This may not be related to the 1.9 Prerelease 3, but I randomly generated a new world without using any seeds and found these two strange one block glass towers; they extend all the way to the tallest block and likely extend all the way to the lowest.
Anyone know what these might be or is this some kind of glitch?
EDIT: I dug straight down in one of the pillars and ended up in a stronghold room with one of those Silverfish spawners and that new glowly blue portal thing, and I dug straight down in the other pillar and ended up in another part of the same Stronghold. Both pillars stopped once it hit a "stronghold type" block, like the stone bricks, cracked stone bricks, and mossy cracked stone bricks.
EDIT2:
This video appears to have encountered the same pillars that I have, and they mention that it ties with a dungeon being underneath it.
http://www.youtube.com/watch?v=SUaLtwcQwJ8