Development-sources should provide "virtual/alsa". See also bugs #14907, #28412. #25020 and #42294. I don't know, which ebuilds don't provide this virtual. I know that sys-kernel/development-sources-2.6.7 don't do, so I had to manually add this to the virtual file.
Steps to Reproduce:
This should already work, as the kernel-2 eclass provides this virtual
How are you determining that this is not happening?
Ok, you're right. I think I've had the problems because I injected development-sources-2.6.7. I re-emerged the ebuild now it's fine.
Great, will close this.