Ray Ozzie has published a set of weblog guidelines for employees.  We don't have some of the concerns he mentions (like SEC issues), but much of it is directly applicable.  I've stayed away from anything this specific, but I don't see anything here that is too onerous.  The one issue I do have is this:

Ozzie's guidelines are very clear that all weblogs are treated as private by an employee who does not represent the company.  I think that's entirely reasonable for public weblogs.  There need to be official spokespeople.  However, in a more inwardly focusing weblog, the product manager for a specific product of the engineering team lead for a project is clearly speaking as an expert on that product or project and should be seen that way, not through layers of disclaimers.