uClibc has supported libstdc++since v0.9.11, and I've used it without problem under m68k uClinux.
EDIT: Perhaps I should've clarified.... you have to re-build libstdc++ against uClibc yourself. It's not difficult.
http://forum.niosforum.com/work2/style_emoticons/<#EMO_DIR#>/smile.gif Also, some quick googling turned up another alternative as well:
http://cxx.uclibc.org/ (
http://cxx.uclibc.org/) which is a small libstdc++ replacement. (incomplete, but seems to be together enough for the basics, like streams) I'll actually probably end up building libstdc++ for nios-II soon myself, so I'll let you know if there's any caveats.