Comcast and NAT

In another post I commented that I had trouble yesterday with what seems to be Comcast blocking ports 21 and 80 so I couldn't serve ftp or http. I mentioned this to my brother who said he remembers reading somewhere that Comcast is "going after people running NAT" but he didn't remember any more. Has anyone else heard this? How would NAT be detected with SuSEfirewall2 on 9.0? Damon Register