it seems ck-sources-2.6.8.1-r9 doesn't provide virtual/alsa and virtual/linux-sources, because portage wants to emerge gentoo-dev-sources and alsa-driver.
ck-sources inherits kernel-2.eclass which sets a provide of "virtual/linux-sources virtual/alsa". Please open a bug with the portage team if this bug is still in the latest 2.0.51 rc version.