there are quite a few ebuild that check /usr/src/linux/.config and possibly need to be recompiled either when it changes or when a new kernel is built. I'm suggesting that an opion be created so that whenever you change your .config you can run an emerge that will update only these builds.
In the case of third party kernel modules, try modules-update as root. In any other case, we would need to add yet another metadata tag stating that the ebuild not only needs the linux sources (which is generally linux-* eclass) but also that when the sources change the program needs to be recompiled. And that IMHO, will not happen.
