Yes. On Leopard and Tiger, the default universal_archs are i386 and ppc. libcxx installs itself universal by default on Snow Leopard and earlier.
It is also expected that you cannot build libcxx on PowerPC at this time. It just doesn't work there yet. If a port you're trying to install or update is pulling in libcxx, then you can't use that port on PowerPC right now, sorry. This will probably apply to a lot of ports.