GCC 4.1 and Glibc 2.4 going stable on x86 and amd64
This means you will have to rebuild your systems. No way around it, because this will change the ABI in GCC, and could render your system completely unstable because of it.
The GCC upgrade guide:
And once again, yes, you MUST rebuild your system. I can not stress this enough. If your machine is slow, you must endure this update, unless of course you wish to remain at your current GCC, then you must mask the upgraded versions. If you upgrade, then you must recompile your system.
GCC 4.1.1 and glibc 2.4 going stable
As you can guess by the title, the Gentoo base project has announced that sys-devel/gcc-4.1.1 and sys-libs/glibc-2.4-r3 will be going stable on both amd64 and x86 this week because of their use in the upcoming 2006.1 release. The ppc architecture has already switched, when using the 2006.1 profile. Other architectures are working on support and will switch over at a later date. This is a major milestone for Gentoo as it marks a transition from the previously deprecated LinuxThreads to Native POSIX Thread Library (NPTL) for glibc. Support for the older versions of glibc with LinuxThreads will remain for x86 in a limited fashion. However, upstream glibc will no longer support LinuxThreads, so bug and security fixes are unlikely. The white paper (PDF) has more information on the design of NPTL. There is a well-written comparison of LinuxThreads and NPTL on IBM DeveloperWorks.
Users on x86 wishing to continue using LinuxThreads are asked to switch to using the "no-nptl" profile for default-linux, or to switch to using a hardened profile.