Using the current gentoo sources it is impossible (afaik) to but a SATA harddisk in standby/suspend/poweroff mode. A simple patch from the libata upstream branch fixes this. I don't know how long it would take before it enters linux-2.6 but it would be really nice to have this feature in gentoo-sources.
*** Bug 102753 has been marked as a duplicate of this bug. ***
Please get this accepted upstream first, then it will appear in gentoo-sources. The libata maintainer is pretty good at sending patches to Linus, so you can possibly assume that there are some problems with this patch.