Take a look at this discussion:
http://thread.gmane.org/gmane.network.wireshark.user/928/focus=928
It's about the same problem and also has a perl-script that looks like
it does what you want.
On Thu, 11 Jan 2007 05:26:29 -0600, "Andrew Chalk"
<achalk@xxxxxxxxxxxxxxxxxxxxxx> said:
> I have a dump of several hundred UDP packets. Can ethereal create a dump
> of
> just the data portion of these packets?
> 
>  
> 
> Many thanks.
> 
-- 
  Hans Nilsson
  hasse_gg@xxxxxxxx
-- 
http://www.fastmail.fm - Faster than the air-speed velocity of an
                          unladen european swallow