I happen to agree with KarlTH and Reptileguy.
From what I've read so far, there are 2 main points people are making in their opposition to KarlTH's and Reptileguy's statements. Though these points are related they are by no means the same and I feel it is important to clarify.
1. Goldfish should not live in a tropical community.
2. Goldfish should not live in warmer waters.
For the interest of the goldfish I would have to agree with point #1, though I am guilty of keeping a goldfish with my angels because my niece brought it home from the county fair and has no tank and her parents wanted to flush it. Anyway, at least from my viewpoint, it seems that the statements made by Karl and RG are not suggesting that you should keep goldfish with tropicals. Just that the reason why you shouldn't keep them together isn't what most believe (temp, dirty etc.).
Point #2 that goldfish can't last long in higher temps is absolutely false. If that were true most people would not be able to keep goldfish in the warmer states. I've said this quite a few times before, my tanks unheated will stay in the 80's during summer and fall. Most of the rest of the year it will stay in the low 70's. Even as I am typing at this moment all my tanks are currently 82-84 degrees unheated and its the middle of september. I doubt most people keep their AC on in the house all day long, let alone buy a chiller for their goldfish tank. Not to mention all the people, including my neighbor, who have an outdoor koi and goldfish pond. Yes it is true, living is not thriving, but where is the line that distinguishes the 2 in this case? Also considering what happens to most goldfish comming from these big chain stores, I would say living is a goal in itself, though again this could be applied to many other fish. To me a goldfish in a warm tank in the hands of an experienced fish keeper is much better off than a goldfish in a coldwater tank in the hands of a person who sees fish as a cheap decoration rather than a living animal.
As for the whole natural biotope thing, you can only get so far. Once you put a fish in a glass box, things are already unnatural. Just my 2 cents.