Since the last update dhcpcd automatically launches wpa_supplicant when it is started.
When you already use wpa_supplicant with the provided init script you suddenly have two instances of wpa_supplicant fighting over the interface, which leads to connections that don't last longer than 10 seconds.

Since I did a bigger update I wasn't sure, what was causing the problem so it took me hours trying to find out, what was wrong with my wpa_supplicant or my iwl5000-ucode - I never got to the idea, that dhcpcd might be the problem until I finally managed to connect with almost everything regarding network disabled and it broke again after adding dhcpcd.

I suggest therefore to at least create a warning after the update (elog, etc.) that you should do "eselect rc delete wpa_supplicant" or to remove the mentioned feature from dhcpcd, since it doesn't seem to be the funtoo way to launch wifi everytime you launch dhcpc.

The feature hook is currently placed in /lib/dhcpcd/dhcpcd-hooks/10-wpa_supplicant