It appears that low privileged users can invoke the LOAD extension to load arbitrary libraries into the postgres process space. On Windows systems this is achieved by calling LoadLibrary (src/backend/port/dynloader/win32.c). The effect of this is that DllMain will be executed. Since LOAD takes an absolute path, UNC paths may be used on Windows, thus a low privileged database user can load an arbitrary library from an anonymous share they have set up, escalating to the privileges of the database user. I am still investigating the impact on Unix.
(this vulnerability was born out of a discussion on #postgresql between myself, lurka and dennisb).