Unfortunately, there's no notion of group in the 3D view, the reason why there is no information about them in the OBJ exported file. And even if this information was added, then there would be some problems to handle groups containing subgroups, because the OBJ format proposes the [font=courier new]g[/font] tag but there's no end tag for a group.
By the way, I looked at the patch proposed by sjb007, but the changes he proposes are really too specific to his needs.