A common question I get asked is how Twitter fits into business, so I got the crayons out and drew the below.

In short Twitter fits into the above and many more points of contact within an organisation, the real question is how best to engage with it. Most companies struggle with this, as it is a move from traditional controlled communications to open interacting conversations that go beyond the brand message.
So plan and offer guidelines but be weary of build policy to control Twitter and other Social Media channels, as you will start to impede on the freedom of speech with in a company and detract from the personal engagement Social Media channels such as Twitter can offer.