The port number issue is interesting. I've seen situations where the same (src ip, src port, dest ip, dest port) are reused within 3 seconds of the FIN from the first connection coming past. A large corporation NATted behind a single IP. Luckily the sequence numbers are far enough apart to avoid problems, but I'm sure it's violating some RFC.
My experience with IPv6 has been pretty good, but it's a biased view since I haven't managed to get Firefox to open an IPv6 connection yet. Everything else works pretty good (except there's so few IPv6 sites around).
I'm disappointed the ISPs are so reluctant to do anything to help, and surprised that consumer equipment hasn't just worked around the problem using automatic 6to4 tunnelling.