No in 1.6.6 it only spawns on coordinates where the x=z.
That rule isn't right. The one patch of clay I found in a 1.6 generated world was kind of near a spot where x=z but none of the clay blocks were at that co-ordinate. Clay is just really, really rare. My world is now nearly 50mb and I found enough clay to craft 10 brick blocks. Had to use Cartograph to find it.
Notch has already acknowledged the glitch on his Twitter so I hope it will be fixed by 1.7 update.
I don't know if pumpkins are as rare as clay in 1.6.x (x=z rule), but atleast they are not spawning where they used to in 1.5_02 and earlier. But I haven't find a single pumpkin in 1.6.x generated chunks, so it seems they they the fate of the clay.
yesterday I started a new world, turned 90 degrees to the left and saw a patch of 10 pumpkins.