Well if you are using the DNS Forwarded you could add a rule for that 
domain that would map it to a different IP address ala
  www   	 whatismyip.com   	 10.10.10.1   	

That is of course a big hack.  Not sure exactly what you are trying to 
prevent, is it just for this site or getting your public IP in general?

If the user wanted to get your external IP and he knew a little of 
anything they could get it.  There are several other sites that tell you 
your IP, or you could send mail, or ssh out, etc.

Louis

Mr. listman wrote:
> Hi all,
> Do believe that is not specific to m0n0wall, but any reponse are welcome,
> 
> Is it possible, to add a rule to mono, telling it that if some one word 
> to surft to www.whatismyip.com, that it would not be able to obtain my 
> public address,
> some kind of iip faking, or so.