Here I can see that skype is stable for amd64: http://packages.gentoo.org/package/net-im/skype But Skype is masked at 64bit profile. Why? Reproducible: Always
$ eselect profile show Current make.profile symlink: default/linux/amd64/10.0/no-multilib
Because skype is a 32 bit application which requires multilib to run on amd64 systems.
What he said ;) It is some binary blob of proprietary mess from upstream. %% file /opt/skype/skype /opt/skype/skype: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.4.1, stripped