On this page: http://gentoo-wiki.com/HOWTO_ALSA_Complete_(includes_dmix)#Kernel_2.6.16 it is claimed that with a 2.6.16 kernel, built-in ALSA should be used, not the externals alsa-driver ebuild. But the ALSA version in 2.6.16 is 1.0.11rc2, and there's no such version for alsa-lib in portage. So it's hard to follow the advice.
Gentoo does not maintain gentoo-wiki.com not review it's correctness. Use the official docs as well. http://www.gentoo.org/doc/en/alsa-guide.xml