Just to add to the fun - furfly's port of the 3.x adserver to ACS Classic 4.x is available from
http://www.furfly.net/downloads. We're not using it in production, but someone (whose name escapes me at the moment) sent me some fixes a while back (which are included) so I guess he's using it, hopefully successfully.
We don't have time to port it right now but if anyone wants to, be my guest.
