Diego Novillo, one of Google's lead GCC developers, proposed on the project's mailing list of Switching to C++ by default in 4.8. "We would like to start the process to make GCC 4.8 build in C++ mode by default. The mechanics of the change are simple enough. I volunteer to test changing the default on all primary targets (assuming I can get them from the GCC build farm)."
On the GCC Wiki they have begun to document their recommended C++ coding conventions when working on GCC in C++ rather than C.
As far as why this move is happening, read GCC Compiler Migrating To Be More C++ Based.
Another hot discussion item right now on the GCC mailing list is also the possibility of setting -Wall by default, but that's being looked at as a very contentious item. That discussion thread can be found here.