Paranoid as I am, I wish there was a way to have emerge virus-scan downloaded distribution files before starting installation. Usually I do this by hand, but I think this would be a very nice option to have available. I realize that there are few virus threats now, but in the event the imaginary threath become real, with this feature Gentoo would already be prepared. Perhaps a make.conf option would enable this, with an exclude-list (e.g. clamav always barfs on its own distribution due to the included test-files). This could possibly be generalized to a pre-/post-processing chain of commands, before actual deployment into the system namespace. Just my $.2
See net-proxy/havp and net-ftp/frox