Comment 1 for bug 148994

There's another rule: the distro is divided in components such as main, universe, multiverse, and restricted (and commercial). Packages in main must build against main and main only; packages in universe may build against universe and main.

Until feisty, libquicktime was in main while the packages you mention (x264 etc.) were not, so it wasn't possible to build the main libquicktime source package against them; now the situation is different in that libquicktime is in universe, but x264, faac, and faad2 are in multiverse.

What could be done is having a second libquicktime package, for example libquicktime-multiverse which would be a copy built against the packages from multiverse.