For some reason everytime I would plug in my USB headset, KDE would say my sound cards are no longer working, "permanently remove? ". I accidentally hit yes, now I don't have any sound and my sound cards don't appear on the phonon list.

I've logon as root and deleted /var/temp/<user>/phonon/hardwaredatabase

but that didn't fix it.

try renaming ~/.kde4/share/config/phonondevicesrc or ~/.kde/share/config/phonondevicesrc depending on where Gentoo puts things then restart KDE

.kde4/share/config/phonondevicesrc is the right place on Gentoo -- I confirm.

This should not happen, and looks more like a kernel bug or missing modules.

After plugging the headset, what do you get at dmesg? Also what's the output of lspci and lsusb?