This is because it supercedes the pkg_postinst inherited from the linux-mod eclass. Patch to ebuild below.
Created attachment 112661 [details, diff] virtualbox-modules-1.3.6.ebuild-depmod.patch Patch to call parent pkg_postinst.
Comment on attachment 112661 [details, diff] virtualbox-modules-1.3.6.ebuild-depmod.patch (oops, mispelled "modules")
This has been fixed in 1.3.8.