Should accountants be blogging? My answer is an emphatic YES!, but you should also approach the subject cautiously.
In my early years in accounting, our firm produced newsletters. The tax one sticks in my mind, not just because one of the students' jobs was to run them over to client offices personally with each major tax change, but also because it was written by someone whose humour and point of view came across clearly. He injected his personality and experience into his writing. The result was that you felt like you knew the writer and would be comfortable talking to him about tax issues.
That would be my advice to a firm that wanted to start blogging: find someone on staff who is writing already (i.e. they have experience with and even enjoy putting their thoughts down coherently as well as working to a deadline). One of the problems I face is consistently coming up with new material.
If they don't have direct experience with blogging, then start by reading the Top 50! Make some comments on the posts. People do read the comments. Also, participating through other people's blogs gives you an entrance into the community. All of the bloggers I have met online have been helpful with the technical points of blogging as well as blog promotion. I get lots of ideas from other blogs.
Before starting a corporate blog, start with a personal blog for a few months, so your writer can get some risk-free experience. I would stay away from an external consultant who offers to write your blog for you. You want the voices of the experts in your company to be heard. Engaging a professional to help you promote your blog or to fit it within the context of your other marketing initiatives is a great idea, however.
Finally, be prepared to start small. Even if all the blog does is initiate a conversation with your existing clients, that alone is worth the effort. If you are already on this path, please leave me a comment about your experiences. At the end of the day, your firm's voice is the voices of its partners, consultants and staff. Blogging will help you get some of those voices heard.