Guys, I tried to do some more experimenting with the Vista clients (fresh install of Vista), and this is what I found out:
- the "avast! Antivirus Service" entry in the firewall exceptions list DOES get created (as far as I can tell - see attached screenshot)
- however, out of the box, the "File and Print Sharing" exception is not checked, and it seems to be required for the communication to actually take place.
So, the solution I have found is to simply enable the File and Print Sharing exception in the Vista firewall, and that's it.
We will try to find out why this is the case (I wasn't aware of the fact that F&PS is used for anything WHS-related in avast) and remove that requirement.
BTW I'm testing this on Vista Business - there's a chance that the firewall presets in the different Vista flavours are different...