Yahoo! has delivered last week its Employee Blog Guidelines :
Basically, just about anyone can and will read what you publish. So keep that in mind. If you’re worried about what your Mom, manager, ex-coworker, or Terry Semel would think, listen to that instinct. And realize that once a cat is out of the bag, you can never get the damned thing back in. The blog world is incredibly efficient at spreading rumors, secrets, rants, hyperbole, and your mistakes.
You’ll make mistakes. We all do. Just try to be smart about it.
But above all else, have fun with it. You’ll meet a lot of interesting people and find that you never think about this company the same way again.
(…)
Go forth and write. Be yourself. Speak your mind.