I recognized that in the minecraft code with version 1.4.4 there is a change in that when placing double slabs the damage value of the block is set to zero regardless of its value before placing.
In the code (I looked at the code with MCP) there is logic for handling the damage value for single slabs on placing, but not for double slabs, its just set to zero.

When one wants to place a double stone slab with id 43:6 it gets changed to 43:0.

This was not the case upto version 1.4.2.

As the block 43:6 is part of the vanilla minecraft I consider this a bug/unintended side effect of this new function handling the damage value on placing slabs.

I am aware, that one can not craft the block with 43:6 in the vanilla minecraft. But why should the code change the damage value, even if the block was crafted with an additional recipe added with a simple mod?

Possible solution: The function setting the damage value to zero for double slabs gets the damage value as a parameter. A simple fix for the above issue is to replace the zero with the damage value passed as parameter. Then the double slab is unchanged on placing. I verified this with the MCP.