While I think Kent Newsome was right when he wrote: "Blogs are like cars- they create a false sense of invincability that releases your inner asshole .", I also think that not all people are assholes, inner or otherwise.
If a blogger needs a code of conduct to treat people with civility and respect, then likely that person's problem runs deeper than any code from on-high will address.
Plainly, if you act like an asshole, even anonymously, then almost invariably, you are an asshole - at least to some degree, despite what your alter ego might project.
Conversely, if you consistently treat people with respect and civility, chances are you are not an asshole.
I fall into the latter category (you'll have to take my word on that). But life has taught me that I might not be in the majority. ;)
When I started blogging I made a couple of mental notes: I would write what I thought and own what I wrote. The second part of that means looking forward and determining whether or not you'd be proud of what you're about to publish, whether that is a blog post, comment or forum posting.
Maybe that is *my* code of conduct.. mehh, it's enough for me.