When I /give myself seamless double slabs (43:6) they look fine as items, but when placed they look exactly like normal double slabs.
What I expected to happen was...:
I expected to be able to place these blocks as I used to in 1.3.2, so I used /give tbh1138 43 1 6. The block looked exactly as expected while I was holding it.
What actually happened was...:
I placed the seamless double slab, and instead the game acted as if I had placed a normal double slab. It's more annoying to build platforms out of seamless slabs now that I have to use individual slabs instead of double slabs.

Steps to Reproduce:
1. Load any world with cheats or join a multiplayer server.
2. Type /give @p 43 1 6 or get the admin to type /give (your username) 43 1 6.
3. Place the block received.
4. Watch as the seamless slab magically gets a seam halfway through and turns into two normal slabs.