Heya, since there are testificates that sell things now at the spawn I figured that perhaps they should sell special blocks that cannot be obtained normally such as the double stone slab with only the top texture. The only way to get it in minecraft at all is if you have cheats on in single player and do the command /give *playername* 43 (ID for double slabs) 1 (quantity) 7 (the secondary ID or whatever it is which determines which slab it is. 8 is the normal ID for the double stone slab and 7 is the unused one)
Since in 1.5 there will be a sandstone version of that I was reminded of how great these blocks are for building. (Maybe have it trade slabs in 2 slots for the same amount of the special slab?)

If this gets implemented I'm sure I'd not be the only one pleased =D