There are a *lot* of blogs being published from inside Microsoft - and Microsoft certainly has more going on in this field than anybody by far.
Within Microsoft, the Office team has the most and best blogs. Jensen Harris of the Microsoft Office team has one of the best (along with Chris Pratley of OneNote), and he posts regularly. It's little secret that a new beta of Office 2007 is coming sometime soon. probably this month. and if you aren't signed up for it you should do so now. Office 2007 (as I've discussed on my own blog before) has been updated in several really great ways. including a major interface change (100% different) for Excel, PowerPoint, and Word.
If you a power user of these programs (as I am) you'll want to take a look at their new function and appearance. I've been running them myself for several months now and have already replaced my Office 2003 installs with the 2007 beta. It's that good, I wouldn't go back. Jensen remind us in his morning post to register for the 2007 Office beta here: http://www.microsoft.com/office/preview - click the button labeled "Register Now".
Reference: Jensen Harris blog: http://blogs.msdn.com/jensenh/default.aspx and feed: http://blogs.msdn.com/jensenh/rss.aspx