The FSF's stated position is that you can only link against closed libraries because of this
section of the gpl:
"However, as a special exception, the source code distributed need not include anything that is
normally distributed (in either source or binary form) with the major components (compiler,
kernel, and so on) of the operating system on which the executable runs, unless that component
itself accompanies the executable."
The FSF's belief is that without this clause, a GPLed application can only be distributed if the
libraries it links against are also available under terms compatible with the GPL. The
GPLv2/LGPLv3 incompatibility follows from this - LGPLv2 also includes restrictions that are
incompatible with the GPLv2, but LGPLv2 explicitly allows you to use the code under GPLv2
instead. LGPLv3 only allows you to fall back to GPLv3, which itself is incompatible with GPLv2.
Now, the fact that companies like Sun and Apple distribute GPLed code linked against their non-
GPL compatible system libraries as part of their default install (and so would seem to trip over
the "unless" component of the above) indicates that they appear to have a different belief about
what a derivative work is. I don't know of any cases which have actually tested this, and I
suspect it's in the FSF's interests to leave it as a grey area for the moment.