Are Dark Templars visible during warp-in if you have warp gates?
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:
According to this they cloak while warping in.