A simple example: it is totally trivial on Windows to build a 'service' from a DLL, exposing its entire interface. This would be running as a separate executable, but would look like a regular library to any windows program.
The FSF's position is that the GPL applies to any programs which are designed to link with that DLL, that this is legally equivalent to statically linking them. -- license-discuss archive is at http://crynwr.com/cgi-bin/ezmlm-cgi?3