The defaults are already pointing to the GCC 4.9 components for GDC, GCC Go, GCC Java, and Gnat (Aada) front-ends on all architectures while the GCC 4.9 default for C, C++, Objective-C, and Objective-C++ front-end handling is a few weeks out. The Fortran support is also in the process of moving to GCC 4.9. When these changes land within the Debian archive, they'll be picked up within Ubuntu Linux, well in time for Ubuntu 14.10.
Matthias Klose has been leading when it comes to preparing the GCC 4.9 support for the Debian/Ubuntu world. Overall, the GCC 4.9 supp;ort is looking good with few issues to resolve. GCC 4.9 is expected to become the default compiler for the C/C++ languages by the end of May or start of June.
More details on preparing GCC 4.9 for Debian and Ubuntu 14.10 can be found via this mailing list post.