I noticed you can't use the walkable tiles for null terrain anymore. What is up with that? Did it move to a different location on the palette or am I doing something wrong? I'm using the ones by the creep. According to the wiki those next to the creep are the walkable null terrain for all ground units.

I think you can't create or preplace units on null terrain, but you can drop a unit onto it via dropship/reaver/overlord.

You can't walk from normal walkable terrain onto that terrain, but you can create/teleport units on that terrain and they use that terrain like it is walkable for them until they leave it.

I figured that was the answer when i read the wiki a little more carefully. But still, you used to be able to place the unit onto the terrain when editing the map. I've done it before, that's why I was curious. Thx

Look in your unit placement options. You may have "Place Units Anywhere" turned off.