This article in today’s Grauniad is well worth a read for all exhibition and conference organisers and indeed any naysayers who (still) don’t necessarily see the value of Twitter at industry events.
‘SXSW: Twitter proved to be more than a social network: Some valuable lessons from SXSW’s interactive festival‘ can be read here 
A couple of points in the article that we would particularly highlight…
“Someone walks out of an interview session frustrated by the interviewer, and only the people in the room feel the energy shift. Tweet the message … your own followers see it, possibly retweet it … respond. People watching the relevant hashtag, …. start to see patterns”
“The #futureofcontext hashtag also helped the conversation carry on past the session. Backchannels aren’t new … Twitter simply makes it more accessible .. It also broadcasts the results further and can be searched”
You add screens running TwitterFall  or similar that we did at #ISE2010’s #DOOHBizConf and you also of course bring in the digital signage element and have a seriously powerful ‘in event’ element as well.