For those interested in the upcoming C++0x support, which has been worked on now for several GCC releases, there continues to be greater support with the 4.7 code-base. GCC 4.7 has improved experimental support for the C++0x ISO standard. C++0x was just approved by the ISO and will likely be known properly as C++11.
There's also other improvements in GCC 4.7 for ADA, C, C++, Fortran, and the libstdc++ run-time library.
For those dealing with multi-threaded code, GCC 4.7 comes with support for OpenMP 3.1. OpenMP 3.1 is supported by GCC 4.7 for C, C++, and Fortran compilers.
A number of old systems/targets have been obseleted from GCC's code-base as well, and they will be removed with GCC 4.8 unless work on them is restored. Targets include IRIS 6.5, MIPS OpenBSD, Solaris 8, and Tru64 UNIX v5.1. In GCC 4.7, support has been removed for NetWare x86 and Unix International Threads on Solaris 2 along with the Solaris BSD Compatibility Package.
New to GCC 4.7 is support for Texas Instruments C6X processors along with some PowerPC architecture improvements. However, at this time, there aren't any major target-specific improvements. We're still waiting to see optimizations for AMD Fusion Llnx/Llano hardware, among other recent CPUs.
The current change-log for GCC 4.7 can be found on this GNU.org web-page.