I actually started writing a long post about how linking agains shared libraries works and what and how ld.so does, but that seemed to get very long. So here the short version that assumes pre-existing knowledge.
If you want an RPATH in your DSOs (dynamic shared objects: executables and shared libraries) you can pass in -R/-rpath to the linker (the runtime_library_dir keyword argument to distutils's Extension class). This is not always very practical though, e.g. when building Python it would be a major pain to modify all the makefiles and setup.py. Error prone too.
So the linkers (both GNU and Solaris) also accept an environment variable: LD_RUN_PATH. When that is set it is used to populate the RPATH field, but only when no explicit -R/-rpath is specified. So far so good.
On Solaris however your gcc will usually not be installed in the root system, rather in /usr/sfw (sun supplied) or /opt/csw/gccX (opencsw). So the gcc libs will also not be in the default library search path. But gcc is nice and helps you out, it will implicitly add a -R option to ld pointing to it's own library directory (for libgcc). Now I'm not sure how nice exactly this is since it screws your LD_RUN_PATH environment variable over and you actually need to run gcc traced to see this happening, have fun finding that! It would be nice if gcc would extend the environment variable if you had it set but where using no -R flags instead. Oh well, at least now you know.