Here's an article in CIO Insight on guidelines for blogging in a corporate environment - sounds like a good set of tips to me. The most important one (from Sun's guidelines, apparently):

The worst thing that can happen is that a Sun sales pro is in a meeting with a hot prospect, and someone on the customer's side pulls out a printout of your blog and says "This person at Sun says that product sucks." Using your weblog to trash or embarrass the company, our customers, or your co-workers, is not only dangerous but stupid.

This is one of the reasons I wonder about some of Scoble's posts. Plenty of people call him "courageous" for standing up to management at MS - but there's a huge downside as well. The old adage about honey and vinegar applies here.