So, to make it more visible, here's a link to that:
How to work with external forms (external SWF files) in Arp.
The code in there is for Flex 1.5 but the technique is the same regardless of whether you're using Flash, AS2, AS3, Flex 1.5 or Flex 2.
The Registering external forms (SWF files) in Arp article by Aral Balkan, unless otherwise expressly stated, is licensed under a Creative Commons Attribution-Noncommercial 2.0 UK: England License.
arp 3? how do I get my hands on that?!
Hey Paddy,
Your best bet is to check out the latest version from SVN (http://svn1.cvsdude.com/osflash/arp). There is no official Arp 3 release (yet).