Do comentário do OP:
I found the root cause. I'll share it here for anyone who hits the same problem. The problem was that Sabayon ships with ufw (a firewall, it seems) per default. For some unknown reason, this failed to initialize properly. Doing # rc-update del ufw and rebooting fixed the issue for me. – Jannik Jochem Dec 28 '12 at 15:54
Com isso, a pergunta é efetivamente respondida (e, como efeito colateral, removida da fila de Perguntas não respondidas).