I can see two possible sides to ethics. 1. Being ethical with content. 2. Being ethical in business.
As for content, I agree with Cole's sentiment. There are too many different tastes in humor and interest to set standards. The web is a place that can accompany all. Now, I don't think it's a good idea for comics (or any medium) to encourage hate groups or racism, etc. But cartoonists have a way of pushing the envelope with content. That's an important part of our industry and I see no need to stifle that.
As for business ethics, I think it's very important. But with the current popular model of each cartoonist running their own small business, it's difficult to track/implement. The ways cartoonists make money online, i.e. product stores, advertising, licensing, does not lend itself to poor and unethical practices. These are normal systems already set in place.
If there were some "official webcomics guild", sure, that group may compose some guidelines of sorts. But that doesn't exist and I don't think it's what this community is about.