The AHCI/SATA controller provided by the VIA VT8251 southbridge is still not recognized by the kernel AFAIK. IDE support is OK. Upstream request.
This is a bad place to request kernel features. http://bugzilla.kernel.org/
Sorry I was unaware of the Gentoo bugzilla etiquette. Also, I thougth that other Gentoo users might want the same. Anyway, I have filed the request upstream.