Update: I jumped through mind-boggling hoops and managed to get rid of
the double libstdc++ stuff. I'm sure it was a bad thing that I was
compiling qtruby with g++ 4.0 and qt had been compiled with 3.3. I've
ironed all of that out now, qt and qtruby and smoke and everything has
been compiled with g++ 4.0. Unfortunately that didn't fix the problem.
:(

Here's the new ldd of qtruby.so:

$ ldd /usr/local/lib/site_ruby/1.8/i386-linux/qtruby.so
        linux-gate.so.1 =>  (0xffffe000)
        libm.so.6 => /lib/tls/libm.so.6 (0xb7f0b000)
        libsmokeqt.so.1 => /usr/local/lib/libsmokeqt.so.1 (0xb7ab5000)
        libqt-mt.so.3 => /usr/lib/libqt-mt.so.3 (0xb72d3000)
        libpng12.so.0 => /usr/lib/libpng12.so.0 (0xb72ae000)
        libz.so.1 => /usr/lib/libz.so.1 (0xb729a000)
        libXext.so.6 => /usr/X11R6/lib/libXext.so.6 (0xb728b000)
        libSM.so.6 => /usr/X11R6/lib/libSM.so.6 (0xb7282000)
        libICE.so.6 => /usr/X11R6/lib/libICE.so.6 (0xb726b000)
        libpthread.so.0 => /lib/tls/libpthread.so.0 (0xb7259000)
        libGLU.so.1 => /usr/X11R6/lib/libGLU.so.1 (0xb71db000)
        libGL.so.1 => /usr/X11R6/lib/libGL.so.1 (0xb7177000)
        libX11.so.6 => /usr/X11R6/lib/libX11.so.6 (0xb70ab000)
        libc.so.6 => /lib/tls/libc.so.6 (0xb6f73000)
        /lib/ld-linux.so.2 (0x80000000)
        libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0xb6e99000)
        libgcc_s.so.1 => /lib/libgcc_s.so.1 (0xb6e8e000)
        libfontconfig.so.1 => /usr/lib/libfontconfig.so.1 (0xb6e5f000)
        libaudio.so.2 => /usr/lib/libaudio.so.2 (0xb6e49000)
        libXt.so.6 => /usr/X11R6/lib/libXt.so.6 (0xb6dfa000)
        libjpeg.so.62 => /usr/lib/libjpeg.so.62 (0xb6ddc000)
        libXi.so.6 => /usr/X11R6/lib/libXi.so.6 (0xb6dd4000)
        libXrender.so.1 => /usr/lib/libXrender.so.1 (0xb6dcc000)
        libXrandr.so.2 => /usr/X11R6/lib/libXrandr.so.2 (0xb6dc8000)
        libXcursor.so.1 => /usr/lib/libXcursor.so.1 (0xb6dbf000)
        libXinerama.so.1 => /usr/X11R6/lib/libXinerama.so.1
(0xb6dbb000)
        libXft.so.2 => /usr/lib/libXft.so.2 (0xb6da8000)
        libfreetype.so.6 => /usr/lib/libfreetype.so.6 (0xb6d3b000)
        libdl.so.2 => /lib/tls/libdl.so.2 (0xb6d37000)
        libXxf86vm.so.1 => /usr/X11R6/lib/libXxf86vm.so.1 (0xb6d32000)
        libexpat.so.1 => /usr/lib/libexpat.so.1 (0xb6d11000)