emerge libkmime would load kdelibs-3.5.1.tar.bz2 and kdepim-3.5.0.tar.bz2 which now is to old as we have kde-3.5.2
No, it's the way split ebuilds works, if the sources hasn't changed, it will still build from the old tarball. It's supposed to be this way.
OK, i have a dsl flat rate and using gentoo makes no sense otherwise, but it has the conseqence that unnecessary distfiles (around 100 MByte) have to be archived or downloaded.