Among the changes talked about already for GCC 4.8 is its greater optimizations, 64-bit ARM support, the Local Register Allocator, improved C++11 support, and other new features.
When it comes to the libstdc++ runtime library for GCC 4.8, the in-development documentation now cites improved experimental support for the ISO C++11 standard, improvements to random, and a --disable-libstdcxx-verbose configuration option for disabling diagnostic messages from abnormal process termination.
The libstdc++ improvements for C++11 in this next GNU Compiler Collection release includes forward_list meeting the allocator-aware container requirements, and the following are defined by default: this_thread::sleep_for(), this_thread::sleep_until(), this_thread::yield().
The random improvements include an SSE-optimized normal_distribution function and random_device now supports using the hardware RNG instruction on newer x86 processors. RNG is present with the new Intel Ivy Bridge processors thanks to its "Bull Mountain" hardware-based random number generator. Also improving the randomness is a new random number engine simd_fast_mersenne_twister_engine with an optimized SSE version and eight new random number distributions (beta_distribution, normal_mv_distribution, rice_distribution, nakagami_distribution, pareto_distribution, k_distribution, arcsine_distribution, hoyt_distribution).
While there's been many C++11 improvements within the GCC 4.8 compiler itself and the libstdc++ runtime library, the support as of GCC 4.8 is still deemed experimental. The current state of C++11 support within libstdc++ is listed within the GCC online documentation.
Other recent C++11 news of interest is More Open-Source Projects Eyeing Up C++11, C++11 Support In Qt 5.0, and LLVM Developers Ponder Using C++11 Features.
Separate from GCC's libstdc++, LLVM's libc++ standard library continues maturing as well too with a focus on C++11 compliance. The latest details on LLVM's C++ library are available from its LLVM.org project site. The brief current status is still displayed as, "libc++ is a 100% complete C++11 implementation on Apple's OS X. LLVM and Clang can self host in C++ and C++11 mode with libc++ on Linux."