While the casual style of blogging is so popular with political and ‘geek’ blogs, business blogs need to maintain a professional tone.

Exceptions:

1. Personal blog that represents a business
   Jeremy Zawodny, of Yahoo!, has a personal blog.  Because he’s a high-level blogger that also holds a high-level job at Yahoo!, his blog has become where you go to get ‘Yahoo!’s position’ on a breaking story.
2. High-Tech Employee Blog
   Often an employee blog that represents a high-tech firm will be more casual because its focus is the ‘geek’ sector and they tend to appreciate casual speech, seeing it as an indicator of transparency.

Jeremy has a blurb on his blog that the comments made don’t necessarily reflect those of his employer.  If your blog could function with a similar statement, then possibly you could write more casually. 

However, most businesses will need to continue to speak professionally.  For instance, if you were a stockholder of Viacom, and you happned to visit Viacom’s blog, you would feel uncomfortable if the writer were discussing office politics, the latest episode of Lost or generally speaking on the company blog as if he was speaking to a group of college buddies.

Consider your market.  If you’ve got a base of geeks or if you are writing on your personal blog, feel free to be entirely casual in tone, but if you represent a company that wants a blog to be their online voice, understand the writing on said blog must be written in a formal tone.