>NAT is a totally different thing.
>UDP is stateless (http://www.faqs.org/rfcs/rfc768.html)

Even through the RFC says it is stateless, there is enough
information so that NAT boxes can do their thing.

If the goal is to make iptables not get confused by
answers to UDP packets, it might be a good idea to
copy whatever tricks NAT is using.

--
These are my opinions, not necessarily my employer's. I hate spam.