Pixel Buffer Objects and Cube Maps

I'm implementing a renderer interface with OpenGL. The renderer uses a D3D style Create, LockRect, and UnlockRect system for cube map textures. I would like to use pixel buffer objects for the glMapBufferARB and glUnmapBufferARB functions, but I'm stuck when it comes to finding the offset into the PBO for any particular face of the cube map. Also, cube maps can have multiple mip levels. Does anyone know how these are stored internally so I can index into the PBO? Thanks!