I am very excited about C++11, and hope we can start using it ASAP. Even a small subset would already be awesome.
I started a page on our wiki http://wiki.inkscape.org/wiki/index.php/C%2B%2B11 to keep track of what new features are safe to use.
Well, which Linux distributions (i. e. versions) should be supported? So we could simply find out the default compiler versions there and look into the according gcc documentation.
GCC 4.8.1 is feature complete. http://gcc.gnu.org/projects/cxx0x.html
libstdc++ is missing some rather important things such as regex support. http://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html
clang 3.3 is feature complete as well. http://clang.llvm.org/cxx_status.html
libc++ is an alternative c++ library that targets c++11 and is feature complete. http://libcxx.llvm.org/
As far as Mac OS X 10.6.8 is concerned I can say that the default compiler is GCC 4.2, which does not seem to support any c++11 features. Anyway, I managed to compile Inkscape there with a brand new clang 3.3 and libc++, so this should be possible for any newer Mac versions as well. The default compiler/library combination on newer Mac versions is clang/libc++.
Btw, could I get a wiki account? I could then update the Mac OS X compile instructions.
Regards, Markus