C++ Modules - why were they removed from C++0x? Will they be back later on?

Clang is the first compiler to start working on modules even before the standardization is complete. There is not much of a documentation yet, but example code could be found here:
http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Modules/

Some comments from Douglas Gregor (the developer implementing them):
http://clang-developers.42468.n3.nabble.com/C-modules-td3619936.html

In theory, you can define a bunch of helper macros like begin_module, end_module, import_module to shield yourself from any likely changes to the syntax that will come in the future.

EDIT 1:
Douglas Gregor has released a presentation about his implementation:
http://llvm.org/devmtg/2012-11/Gregor-Modules.pdf?=submit

EDIT 2:
The module support in clang have been documented here:
http://clang.llvm.org/docs/Modules.html

EDIT 3:
Modules are now supported in Microsoft's C++ compiler as well: http://blogs.msdn.com/b/vcblog/archive/2015/12/03/c-modules-in-vs-2015-update-1.aspx

1. Because it is very big conceptual change.
2. There is no real need of it as separation of the sources to h/cpp does the job
3. Because C++ does not define how actual "modules" libraries are build. It leaves it to compiler developer and to linker.
4. "Modules" are sometimes quite platform dependent, for example DLLs quite different from shared objects. So it is not so trivial to merge between these concepts.