Logic+Emotion points me to the WOMMA Releases Blog Ethics Guidelines:

This document is a public draft of guidelines for marketers to follow when doing outreach within the blogosphere. It is neither a „how to blog“ nor a „what to blog“ document. Rather, its intent is to give clarity and guidance to marketers who are working and corresponding with bloggers, and to ensure that their efforts adhere to the standards set by the WOMMA Ethics Code.
1. I will always be truthful and will never knowingly relay false information. I will never ask someone else to deceive bloggers for me.
2. I will fully disclose who I am and who I work for (my identity and affiliations) from the very first encounter when communicating with bloggers or commenting on blogs.
3. I will never take action contrary to the boundaries set by bloggers. I will respect all community guidelines regarding posting messages and comments.
4. I will never ask bloggers to lie for me.
5. I will use extreme care when communicating with minors or blogs intended to be read by minors.
6. I will not manipulate advertising or affiliate programs to impact blogger income.
7. I will not use automated systems for posting comments or distributing information.
8. I understand that compensating bloggers may give the appearance of a conflict of interest, and I will therefore fully disclose any and all compensation or incentives.
9. I understand that if I send bloggers products for review, they are not obligated to comment on them. Bloggers can return products at their own discretion.
10. If bloggers write about products I send them, I will proactively ask them to disclose the products’ source.

Good to have this summary, even though this really should be common sense, since it means: act honestly and transparently. A basic prerequisite when dealing with people. (In theory.)