There's a package list in the x.org tutorial: http://www.gentoo.org/proj/en/desktop/x/x11/modular-x-howto.xml One package is missing from that list: x11-misc/linuxwacom Modular x.org will not merge correctly because the stable version of above package has autoconf bugs (see http://bugs.gentoo.org/show_bug.cgi?id=101674) when USE=sdk has not been defined. x11-misc/linuxwacom should be added to the package list.
Not a Gentoo infrastructure bug.
Additionally, you are not supposed to mix stable and ~arch packages (generally, not just w/ modular X).
Just to add a "me too" here - 0.6.7 fails to build because it doesn't have the X-modular deps. Need 0.7.2 of linuxwacom in the package list, http://www.gentoo.org/proj/en/desktop/x/x11/modular-x-packages.txt
Added to the list, thanks and sorry for the delay!