I was not holding shift.
Also, in tandem with this, most of these islands do not exist as far as the generator is concerned, the blocks are either invisible, or exist purely as textures, with no solidity to them. Ender pearls can't be used to jump onto these islands, and standing on them will cause you to fall slowly through the blocks, before falling into the void.
Oops, my bad, I got the mouse buttons mixed up, I meant MOUSE1, or the "attack" button.
They do now, see it for yourself in the patch notes on the wiki: http://minecraft.gamepedia.com/15w47b