As you can see in bug 7969 emerge has troubles when using nss-mysql (it is a bug in nss-mysql, I presume, but it only shows up in emerge during the unpacking, regular usage of tar works like a charm).
A possible workaround around this problem is adding --numeric-owner to $tarvars in ebuild.sh. This way, tar will not ask for the user info of the original creator of the tarball (which is were it borks because nss-mysql doesn't seem to answer quick enough). This even produces less overhead because it is one less lookup ;)
Portage queries this info as well. Also, we cannot assume some numbers.