The hotplug policy agent in /etc/hotplug/net.agent expects redhat-style ifup/ifdown interfaces to register/unregister network cards. Gentoo doesn't provide them so the script should be patched. There is a preliminary patch on the gentoo forums, http://forums.gentoo.org/viewtopic.php?t=691 that might be useable as a starting point.
Created attachment 4805 [details, diff] patch to add /etc/init.d/net.* support to net.agent this diff is against hotplug-20020826, but it should apply cleanly to 20020401
Since Chad's list is long, I will take care of this one for him. Look for this fix to appear soon, both in 20020401 and a new 20020826 ebuild.
I have committed hotplug-20020401-r1 which should fix this problem. (I don't have a network device to test personally.) You might also test hotplug-20020826 (which presently requires ACCEPT_KEYWORDS="~x86 ~ppc").