All companies who have blogging sites have rules. Most companies large enough to have blogs have confidentiality agreements as well. On blogging sites, on myspace or any of the millions upon millions of public sites people are exposed to you should not express personal information about employees or private information about your company.
Anyone who reveals sensititive information has not only written there own pink slip, but ruined there name for possible jobs to come. I have heard of employers "googling" to see what they can find out about an idividual before they are put on payroll. If you blog yourself out of your job it will be found if a future employer decides to find it. Since the internet has arrived it seems no one can hide.