ISPs blocking and throttling
"If they are inserting additional packet's into a customers data stream, with the customers IP address etc... it sounds more like forgery or identity fraud than reasonable behaviour"
I'm not sure I'd go that far, I'd only go so far as to call it port blocking. It's still a really major headache though.
All of the cable ISPs I've dealt with (in US) do it.
Time Warner's Road Runner service was horrible about blocking thousands of incoming ports rather arbitrarily such as the entire 6000-7000 range outright which sucked for VNC, remote X displays and several games (ironically port 80 was open).
Cable Vision's Optimum Online blocks several incoming as well as outgoing ports, which sometimes makes it difficult to use legitimate services.
As for throttling, at times I have noticed network bandwidth going down to modem speeds but I've always just attributed them to a "slow internet", although the bandwidth speed tests would show no bottleneck at all. It wouldn't surprise me if it was due to a cap.
The major ISPs love to throw out "security" as the reasoning for blocking traffic, even if we know better.
If they won't change, I think the only solution will be to use random ports with encryption for everything.