What if an interface starts up?

What happens if you bring up an interface using an init script on gentoo? Do you know? Already figured out how to start services depending on what interface is up/down?

The gentoo-way is done in /etc/conf.d/net.

Looking at the net.example config you might find some functions named preup(), predown() postup(), postdown(), failup() and faildown() - those are hooks for events which you may use to call some programs or scripts. That way it is possible to do really fancy stuff.