David Meerman Scott's Social Media Guidelines:
1. Transparency
–Never pretend to be someone you are not
2. Privacy
–Unless given permission, don’t blog about something disclosed to you
3. Disclosure
–Disclose anything people might consider a conflict of interest
4. Truthfulness
–Don’t lie
5. Credit
–Give credit to bloggers (and other sources) whose material you have used in your blog
Pulled from David Meerman Scott's own blog post on the 3M Social Media Debacle. (Worth a read!)