The best of corporate blogging comes when the post isn't directly related to you. One such master of this style is BT's own J.P. Raganswami, who writes posts like this thought-provoking piece at Confused of Calcutta. The post has nothing to do with telecom, nothing to do with new BT products and services, and yet, when you read it, you think positively about JP and his ability to provide useful information.
Blogs can be more closely related to your role, such as Mario Sundar's Marketing Nirvana. Mario works as LinkedIn's community guy, and has lots to say to the business world about the use of social media tools. For example, Mario provides Five Best Practices on Corporate Blogging that proves useful, and this, in turn, gives one impressions about what type of people work at LinkedIn.
In considering your approach to corporate blogging, and what you're trying to accomplish, ask yourself whether you're looking to blog about the specific products and services your company offers, or whether you're trying to provide a resource to the community at large. There's a place for both types of blogs, but only the latter has a chance at reaching outside your existing customer base and providing some value. Understanding your blogging strategy and where it fits into your other digital marketing and communications initiatives will help you derive value from the effort. But, please, before you dive into corporate blogging, definitely consider seeing what's out there in your space. You might find some great new information resources, as well as an idea or two on what the community is interested in talking about.