Mike Damphousse of Green Leads used Twitter to cover the Sales 2.0 Conference in San Francisco on March 4th and 5th 2009. He was able to capture lots of interesting soundbites and links, and interact with other conference attendees, and even people who were not at the conference.
Apart from a Twitter outage and a dead laptop battery, it went very smooth. However, it was a lot of work, and it's hard to really listen to the presentations if you're twittering at the same time.
Mike thinks Twitter is a great tool for marketing your company, and - for sales reps - to do research on the people they are calling on beforehand.