I know that units being warped in can be attacked as soon as their visage begins to appear on the screen. Is this visage cloaked for dark templars?
Resorath and the wiki are correct; Dark Templar indeed are cloaked while warping in.
In this screenshot, the two sentries ignore the Dark Templar during it's warp in:
But once we add a Raven to the mix, the sentries switch their focus to the templar soon after the warp in initiates: