INET_IFACE: Secure to allow traffic from 192.168.x.0/24?

[Carsten Maass <cm@xxxxxxxxxxxxxxxxx>]

Dear List,

recently i connected together two internal networks over an IPSec-tunnel:

(Localnet A)---(Gateway A)==IPSec==(Gateway B)---(Localnet B)

Now i am unsure which iptables-rules i should apply to the external 
interfaces of the gateways to match the traffic between the Localnets 
without opening up a security hole. Is it sufficient to simply apply 
some general rules like:

$IPTABLES -A FORWARD -i $INET_IFACE -s 192.168.a.0/24 -j ACCEPT
$IPTABLES -A FORWARD -i $INET_IFACE -s 192.168.b.0/24 -j ACCEPT

or would this approach be vulnerable to some kind of IP-spoofing attack? 
What would be your solutions in this situation?

Thanks for your help,
Carsten.