The libcxxrt library is meant to be a total replacement to tthe GNU libsupc++ library and to be a further step away from the GNU GPL on BSD. The press release announcing this effort by PathScale with a focus on NetBSD and FreeBSD can be read here. It looks like PathScale is setting their support sights now on the BSD operating systems, "This is a first step to PathScale offering first class support for both NetBSD and FreeBSD."
The Low-Level Virtual Machine developers meanwhile have been developing libc++, which is under the MIT and UIUC (BSD-like) licenses and with this C++ library they are targeting the C++0x standard. This library was written to avoid the mainline libstdc++ library in GCC that is now GPLv3 licensed, to provide performance improvements, and to ensure C++0x support. Right now, LLVM libc++ is currently limited to Mac OS X support.
This BSD licensing of libcxxrt isn't PathScale's first open-source work they have done for the community. PathScale is also the company that wanted to port the Nouveau driver to OpenSolaris and offered free NVIDIA graphics cards to open-source developers. Ultimately they ended up forking the Nouveau kernel driver to create a new PSCNV driver due to different views over memory management and other focuses. PathScale is primarily interested in the open-source NVIDIA/Nouveau driver for eventually empowering GPGPU/CUDA/OpenCL support.