Actually, the FSF and many of its representatives, has claimed, on
many occassions, that the GPL infects across dynamic linking. That
is, if you write your own code that calls readline which links via a
dynamically linked shared library, and perhaps even across dlopen(),
they claim that the GPL applies to the code which you write. Given
that the only way this could happen is via copyright law, they are
basically saying that if you use the readline interface, you have
created a derived work and they therefore 0wn your source code.
Whether or not this would be laughed out of court or not will very
much depend on the local legal precedents (and Trent Waddington has
quoted some very interesting legal cases based on US court decisions,