Among other reasons, I'd guess this is Apple's attempt at modernization. The GCC/libc/libstdc++ codebase has been around for ages, and has accumulated cruft. Starting from scratch isn't always a solution, but in cases like this I think it can be.

Modernizing the compiler (via clang/LLVM) is a great step forward, but modernizing the C++ libraries to take advantage of the new compiler infrastructure is a sensible step 2. If they can in fact speed up compilation speed and memory requirements (both at compile-time and run-time), as well as providing link-time and run-time optimization, it could be a big win, especially for large C++ projects (such as Mozilla, KDE, Qt, etc.).

I would imagine a lot of it involves improving xcode by using the same code for compiler and IDE. GPL2 or GPL3 really doesn't make much of a difference in this regard.

Although the GPL3 question comes up with the non-inclusion of a newer version of Samba in OS X Server. This is making it hard / impossible to add Win 7 to a network with a OS X Server PDC.

I would imagine that's the case. Either way, its nice to see llvm's c++ support coming along.

With in-house processor design (Intrinsity), perhaps secrecy (and anti-jailbreak) is their aim:

In this scenario, they want to be able to ship an XCode and iPhoo SDK without having to distribute the source to their code generator.

i was surprised they didn't base it upon Apache Stdcxx, if they were just trying to avoid the GPL: http://stdcxx.apache.org/