Multiarch cross-dependency satisfaction (as currently implented in apt) needs -dev packages as well as plain lib packages to be make multiarch-ready and thus co-installable. This process is not yet well-specified like the library conversions, and it needs to be for package maintainers to be able to do this work.
A related issue is changing whether the default of apt-get build-dep -a is to install for HOST or BUILD arch. The latter requires making library and -dev packages Multi-arch: same, the former requires marking all other build-deps Multi-arch: foreign. See http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=646288 for discussion, along with the table at the bottom of: https://wiki.ubuntu.com/MultiarchCross
Things to cover: Change apt build-dep -a behaviour?
Guidelines for -dev conversion: header moving binaries (foo-config) what else?