I would surmise that by the presence of an XSI path in your ldd output,
your normal user account uses the LD_LIBRARY_PATH variable, and places the
path to the libstdc++.so.5 that comes bundled with XSI earlier in the path
than /usr/lib; not so with root, which is why it works for root.

It may not be the XSI environment - so don't necessarily point the finger
at that application - but I'd be fairly willing to believe that your
normal user account sets LD_LIBRARY_PATH, and that's the source of the
discrepancy.

hth,
Klaus