I tried to build Umbrello with the minimal set of dependecies, but the one given by the ebuild misses to specify "arts", which provides "the important program mcopidl". I resolved the bug locally but many users may not so it would a great improvement for Gentoo to fix this ;-)
umbrello depends on kdebase which depends on kdelibs which depends on arts
So why "emerge umbrello" doesn't install arts ?
that's a good question, probably a portage bug