Direct3D API is a fully C++ object-oriented API and rely on runtime polymorphisms (virtual fonction calls) to be expendable and easily being able to provide different implementations. So all API calls are virtual calls instead of being plain C calls like in OpenGL.
Every slightly experimented C++ developer knows that virtual functions calls introduce overhead and that they should be avoided inside inner loops. Humus shows how these virtual calls can be replaced by standard calls by hacking the API objects v-table in order to keep a plain C pointer on these virtual methods !
http://www.humus.name/index.php?page=Comments&ID=321

I love this kind of hack ! But as Humus explains, D3D (like OpenGL since OpenGL 3.0) do not rely on immediate mode anymore, that means that API calls are usually consuming a slightly large amount of cycle compared to the overhead of a virtual call.
That means that in practice you wont get significant performance gain from this hack, but this is just really cool :-D And this method could still be useful to overcome performance problems in more badly design APIs !

UPDATE : D3D v-table hacking... made useful !
Humus just published another trick that shows how hacking the v-table of the D3D context can be used to... replace the default API calls by your own enhanced calls !
Humus shows how this can be useful to count the number of time an API function is called for instance. This can be done by overwriting the original object v-table pointer with the address of your own v-table. More details there :
http://www.humus.name/index.php?page=Comments&ID=322