Dell says they can attribute over $1 million in additional sales over the past two years to their use of Twitter (source: Internet Retailer magazine, March 2009 issue). In March of 2007, they started using Twitter to announce sales in their Dell Outlet web store, including exclusive Twitter promotions.
Many companies are using sites like Twitter, Facebook, MySpace, etc. to provide an additional channel of customer service, to promote direct sales, and simply to keep an "ear to the ground" for what customers and prospects might be saying directly about them or their industry, or about developments that could impact them.
In fact, corporate use of Twitter has been profiled in both Business Week and the Wall Street Journal, so I'd say it's pretty close to hitting the mainstream if it hasn't already.
Here's an article profiling 40 major companies and brands that are on Twitter
, including how they use it and what results they've experienced.