In response to a Steam bug report about a libstdc++ problem after upgrading the user's graphics driver and LLVM producing a failure when starting games, Scott Ludwig shared some details about their next Steam Runtime SDK update. In particular, a new SDK update is expected that will support the GCC 4.8 and LLVM/Clang 3.4 compiler stacks (no word about the recently released GCC 4.9, albeit it's not yet found in many distributions). The new SDK is within a chroot environment during the development process to produce a stable and constant build environment separate from the host OS.
Scott also shared they are looking at making GCC 4.8's libstdc++ library the default C++ standard library for the steam-runtime going forward, but to do this they need to do extensive testing of all Steam Linux games on Ubuntu 12.04 through 14.04 releases for ensuring proper compatibility. Additionally, they're looking at adding the ability to mark what Steam Runtime version an app/game was built against for ensuring Steam makes proper run-time environment decisions.
More details can be found within this bug report.