For some reason, this information is difficult to come by.
We'd like to start using some of the C++0x features that are available in most compilers. But we are held back a bit by XCode 3, since it only provides GCC 4.2 and LLVM 1.5. All of the other platforms we need to support either already have compilers with preliminary support for C++0x features, or they have a relatively painless method of installing/upgrading the compiler.
An XCode 4 preview is available, but only to those in the Mac or iOS Developer Program.