Encontrou o problema.
Por alguma razão desconhecida, esta regra de firewall impede que o ucarp funcione.
# Generated by iptables-save v1.4.10 on Thu Oct 6 17:16:01 2011
*filter
:INPUT ACCEPT [22517:2222881]
:FORWARD ACCEPT [2:176]
:OUTPUT ACCEPT [16961:69145734]
COMMIT
# Completed on Thu Oct 6 17:16:01 2011
# Generated by iptables-save v1.4.10 on Thu Oct 6 17:16:01 2011
*nat
:PREROUTING ACCEPT [29:3325]
:INPUT ACCEPT [18:2668]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
-A POSTROUTING -s 192.168.245.0/24 -j MASQUERADE
COMMIT
# Completed on Thu Oct 6 17:16:01 2011