Personally, I have mixed feelings on the subject. Part of me feels that the First Amendment pretty much covers it. Also, there's the "if it ain't broke, don't fix it" part of my brain. But there's also the part that believes a Blogger Code of Ethics could be a good thing simply for our own self-interest. Do we believe our readers deseve to know who we are and where we're coming from? Do we want to gain respect for political blogs? To me, one way to do that would be to VOLUNTARILY develop, adopt, and -- the important but tricky part -- adhere to some sort of standards.
Already, some have made the effort. For instance, CyberJournalist.net has taken the Society of Professional Journalists Code of Ethics adapted it for bloggers. I think it's very interesting. So, here's my edited version of CyberJournalist.net's code, combined with my own ideas as well as Will Vehrs. (By the way, wouldn't this be easier with a Wiki? In the meantime, please feel free to chime in.)
1) Fully disclose. Unless there is some overriding ethical or legal reason why you shouldn't, let your readers know the following information: your true name, your political affiliations and personal agendas wherever applicable, your weblog's purpose and mission, your personal connections to partisan elected officials (if relevant), and any payments you receive related to what you write on your blog.
2) Do not lie or knowingly disseminate false information. If such information is ever published unintentionally, and is found to be untrue, make a correction as quickly as possible, and let your readers know what happened in as open and honest a way as possible.
3) Properly attribute the source of information, photos, etc. -- do not plagiarize. In general, adhere to the same standards you would use if publishing in a prestigious journal in your area of expertise.
4) Do not attack private individuals, including fellow bloggers. For public officials, make sure that whatever you say is factually based, and avoid making statements that can't be supported by any evidence or are completely irrelevant to the issue at hand. For private individuals, avoid going ad hominem if at all possible.
5) Obey Will Vehrs Rule #5. "If I perform the work of a newsgathering journalist in the course of my blogging, I will endeavor to follow generally accepted codes of journalistic conduct, including shielding sources when the reason is explained."
6) Obey Will Vehrs Rule #6 "I will not censor or edit the feedback I receive except for foul language or malicious intent. I will endeavor to be accessible to my readers and to respond to their complaints and suggestions."
7) Have fun. Blogging is not supposed to be painful. Have a good time, just play nice and practice "safe blogging" whenever possible.